diff --git a/src/components/onboarding/__tests__/units-screen.test.tsx b/src/components/onboarding/__tests__/units-screen.test.tsx
new file mode 100644
index 000000000..9dfeb5de5
--- /dev/null
+++ b/src/components/onboarding/__tests__/units-screen.test.tsx
@@ -0,0 +1,82 @@
+/**
+ * v1.39 (Wave C, C3) — Q6 cannot be answered with nothing chosen (M-k).
+ *
+ * "Weiter" used to be live on an untouched units screen: it wrote
+ * `{ units: {} }`, which marked the step DONE without recording a preference,
+ * so the account carried "asked and answered" for a question nobody answered.
+ * Skip is the way past a question with no opinion, and Skip says so on the
+ * ledger.
+ *
+ * Rendered server-side, this repo's component-test convention.
+ */
+import { renderToStaticMarkup } from "react-dom/server";
+import { describe, expect, it, vi } from "vitest";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+
+import { I18nProvider } from "@/lib/i18n/context";
+import {
+  defaultOnboardingSteps,
+  emptyOnboardingNeeds,
+  type OnboardingStateDto,
+} from "@/lib/onboarding/needs";
+
+import { UnitsScreen } from "../units-screen";
+
+vi.mock("next/navigation", () => ({
+  useRouter: () => ({ push: vi.fn(), replace: vi.fn() }),
+}));
+
+vi.mock("@/hooks/use-auth", () => ({
+  useAuth: () => ({ user: null }),
+}));
+
+function state(
+  needs: Partial<OnboardingStateDto["needs"]>,
+): OnboardingStateDto {
+  return {
+    steps: defaultOnboardingSteps(),
+    needs: { ...emptyOnboardingNeeds(), recordTarget: "me", ...needs },
+    completedAt: null,
+    firstResult: null,
+  };
+}
+
+function render(needs: Partial<OnboardingStateDto["needs"]>): string {
+  return renderToStaticMarkup(
+    <QueryClientProvider client={new QueryClient()}>
+      <I18nProvider initialLocale="en">
+        <UnitsScreen state={state(needs)} />
+      </I18nProvider>
+    </QueryClientProvider>,
+  );
+}
+
+/**
+ * Is the primary button's own tag disabled? Read off the tag rather than the
+ * whole page, and off the attribute rather than the string "disabled", which
+ * also occurs in the button's `disabled:` utility classes.
+ */
+function nextIsDisabled(html: string): boolean {
+  const at = html.indexOf('data-slot="onboarding-next"');
+  expect(at).toBeGreaterThan(-1);
+  const open = html.lastIndexOf("<button", at);
+  return html.slice(open, html.indexOf(">", at) + 1).includes('disabled=""');
+}
+
+describe("<UnitsScreen>", () => {
+  it("does not let the person continue without choosing a unit", () => {
+    const html = render({ areas: ["glucose"] });
+    expect(nextIsDisabled(html)).toBe(true);
+    // Skip is still there: it is the honest way past a question with no
+    // opinion, and it says "skipped" on the ledger.
+    expect(html).toContain('data-slot="onboarding-skip"');
+  });
+
+  it("lets the person continue once a unit is chosen", () => {
+    const html = render({
+      areas: ["glucose"],
+      units: { glucoseUnit: "mg/dL", unitPreference: null },
+    });
+    expect(nextIsDisabled(html)).toBe(false);
+  });
+});
diff --git a/src/components/onboarding/units-screen.tsx b/src/components/onboarding/units-screen.tsx
index b2fec48bd..e4be7672d 100644
--- a/src/components/onboarding/units-screen.tsx
+++ b/src/components/onboarding/units-screen.tsx
@@ -54,9 +54,15 @@ export function UnitsScreen({ state }: { state: OnboardingStateDto }) {
 
   const back = previousScreen(state, "units");
 
+  const answerBody = unitsAnswerBody({ asked, glucoseUnit, unitPreference });
+
   async function persist(
     input:
-      ReturnType<typeof unitsAnswerBody> | { step: "units"; status: "skipped" },
+      | NonNullable<ReturnType<typeof unitsAnswerBody>>
+      | {
+          step: "units";
+          status: "skipped";
+        },
   ) {
     try {
       const written = await answer.mutateAsync(input);
@@ -121,9 +127,10 @@ export function UnitsScreen({ state }: { state: OnboardingStateDto }) {
       <StepActions
         backHref={back ? screenHref(back) : undefined}
         onSkip={() => void persist({ step: "units", status: "skipped" })}
-        onNext={() =>
-          void persist(unitsAnswerBody({ asked, glucoseUnit, unitPreference }))
-        }
+        onNext={() => {
+          if (answerBody) void persist(answerBody);
+        }}
+        nextDisabled={answerBody === null}
         pending={answer.isPending}
       />
     </section>
diff --git a/src/lib/onboarding/__tests__/question-config.test.ts b/src/lib/onboarding/__tests__/question-config.test.ts
index 0151f4204..98c14d46a 100644
--- a/src/lib/onboarding/__tests__/question-config.test.ts
+++ b/src/lib/onboarding/__tests__/question-config.test.ts
@@ -80,11 +80,15 @@ describe("questionAnswerBody", () => {
     expect(questionAnswerBody("visit", [])).toBeNull();
   });
 
-  it("sends an empty list for a many-answer question with nothing chosen", () => {
-    expect(questionAnswerBody("areas", [])).toEqual({
-      step: "areas",
-      areas: [],
-    });
+  it("is null for a many-answer question with nothing chosen (I4)", () => {
+    // An empty tick list is not an opinion. It used to be sent as an
+    // ANSWER, and an answered Q2 with no areas is what switched every
+    // optional module off for somebody who meant "no preference" — Skip is
+    // the way past, and Skip is the conservative derivation.
+    expect(questionAnswerBody("areas", [])).toBeNull();
+    expect(questionAnswerBody("sources", [])).toBeNull();
+    // A list of values none of which the step offers is just as empty.
+    expect(questionAnswerBody("areas", ["no-such-area"])).toBeNull();
   });
 
   it("drops a value the step's vocabulary does not contain", () => {
@@ -125,13 +129,22 @@ describe("units", () => {
     expect(onboardingAnswerSchema.safeParse(body).success).toBe(true);
   });
 
-  it("sends an empty answer when nothing was chosen, which leaves the columns alone", () => {
+  it("is null when nothing was chosen, so Skip is the way past (M-k)", () => {
+    // An empty units answer marked Q6 done without recording a preference.
     expect(
       unitsAnswerBody({
         asked: { glucose: true, weight: true },
         glucoseUnit: null,
         unitPreference: null,
       }),
-    ).toEqual({ step: "units", units: {} });
+    ).toBeNull();
+    // One of two asked questions answered is still an answer.
+    expect(
+      unitsAnswerBody({
+        asked: { glucose: true, weight: true },
+        glucoseUnit: "mg/dL",
+        unitPreference: null,
+      }),
+    ).toEqual({ step: "units", units: { glucoseUnit: "mg/dL" } });
   });
 });
diff --git a/src/lib/onboarding/question-config.ts b/src/lib/onboarding/question-config.ts
index 547b28cd0..e3ea93734 100644
--- a/src/lib/onboarding/question-config.ts
+++ b/src/lib/onboarding/question-config.ts
@@ -110,9 +110,15 @@ function only<T extends string>(
 
 /**
  * The body for one answered step, or null when the selection cannot be an
- * answer (Q1 with nothing chosen). Every arm names its field; a value that is
- * not in the step's vocabulary is dropped rather than sent, so the strict
- * server schema never sees a chip this file did not offer.
+ * answer. Every arm names its field; a value that is not in the step's
+ * vocabulary is dropped rather than sent, so the strict server schema never
+ * sees a chip this file did not offer.
+ *
+ * v1.39 (Wave C, C3) — an empty tick list is not an answer either (research
+ * I4). The many-answer arms used to send `[]` and mark the step DONE, and an
+ * answered Q2 with no areas is what switched every optional module off for
+ * somebody whose click meant "no preference". Skip is the way past a question
+ * nobody wants to answer, and Skip is the conservative derivation.
  */
 export function questionAnswerBody(
   step: QuestionStepId,
@@ -123,14 +129,18 @@ export function questionAnswerBody(
       const [recordTarget] = only(ONBOARDING_RECORD_TARGETS, selected);
       return recordTarget ? { step, recordTarget } : null;
     }
-    case "areas":
-      return { step, areas: only(ONBOARDING_AREA_KEYS, selected) };
+    case "areas": {
+      const areas = only(ONBOARDING_AREA_KEYS, selected);
+      return areas.length > 0 ? { step, areas } : null;
+    }
     case "medication": {
       const [medication] = only(ONBOARDING_MEDICATION_ANSWERS, selected);
       return medication ? { step, medication } : null;
     }
-    case "sources":
-      return { step, sources: only(ONBOARDING_SOURCE_KEYS, selected) };
+    case "sources": {
+      const sources = only(ONBOARDING_SOURCE_KEYS, selected);
+      return sources.length > 0 ? { step, sources } : null;
+    }
     case "visit": {
       const [visit] = only(ONBOARDING_VISIT_ANSWERS, selected);
       return visit ? { step, visit } : null;
@@ -171,7 +181,7 @@ export function unitsAnswerBody(input: {
   asked: { glucose: boolean; weight: boolean };
   glucoseUnit: "mg/dL" | "mmol/L" | null;
   unitPreference: "metric" | "imperial" | null;
-}): OnboardingAnswerInput {
+}): OnboardingAnswerInput | null {
   const units: {
     glucoseUnit?: "mg/dL" | "mmol/L";
     unitPreference?: "metric" | "imperial";
@@ -182,5 +192,7 @@ export function unitsAnswerBody(input: {
   if (input.asked.weight && input.unitPreference) {
     units.unitPreference = input.unitPreference;
   }
-  return { step: "units", units };
+  // v1.39 (Wave C, C3) — nothing chosen is not an answer (M-k): an empty
+  // units body marked Q6 done while recording no preference at all.
+  return Object.keys(units).length > 0 ? { step: "units", units } : null;
 }