diff --git a/package.json b/package.json
index 15028861..8608bead 100644
--- a/package.json
+++ b/package.json
@@ -54,7 +54,7 @@
     "prepack": "./scripts/prepack.sh",
     "test": "yarn test:source && yarn test:types",
     "test:source": "jest && jest-it-up",
-    "test:types": "tsd",
+    "test:types": "tsd --files 'src/**/*.test-d.ts'",
     "test:watch": "jest --watch"
   },
   "resolutions": {
diff --git a/src/versions.test-d.ts b/src/versions.test-d.ts
new file mode 100644
index 00000000..d30ed0e8
--- /dev/null
+++ b/src/versions.test-d.ts
@@ -0,0 +1,27 @@
+import { expectAssignable, expectNotAssignable } from 'tsd';
+
+import type { SemVerRange, SemVerVersion } from '.';
+import { assertIsSemVerRange, assertIsSemVerVersion } from '.';
+
+// A value produced via a type's own assertion function is assignable to
+// itself:
+
+const unsafeVersion = '1.0.0';
+assertIsSemVerVersion(unsafeVersion);
+expectAssignable<SemVerVersion>(unsafeVersion);
+
+const unsafeRange = '^1.0.0';
+assertIsSemVerRange(unsafeRange);
+expectAssignable<SemVerRange>(unsafeRange);
+
+// SemVerVersion and SemVerRange are distinct opaque types, and are not
+// mutually assignable, even though both are strings at runtime:
+
+expectNotAssignable<SemVerRange>(unsafeVersion);
+expectNotAssignable<SemVerVersion>(unsafeRange);
+
+// A plain string is not assignable to either opaque type without going
+// through the type's own validation/assertion function:
+
+expectNotAssignable<SemVerVersion>('1.0.0');
+expectNotAssignable<SemVerRange>('^1.0.0');