diff --git a/.chronus/changes/fix-ice-serialize-value-as-json-custom-scalar-2026-2-25-20-0-0.md b/.chronus/changes/fix-ice-serialize-value-as-json-custom-scalar-2026-2-25-20-0-0.md
new file mode 100644
index 00000000000..25affad8bba
--- /dev/null
+++ b/.chronus/changes/fix-ice-serialize-value-as-json-custom-scalar-2026-2-25-20-0-0.md
@@ -0,0 +1,9 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Fix crash when using custom scalar initializer in examples or default values
+  [API] Fix crash in `serializeValueAsJson` when a custom scalar initializer has no recognized constructor (e.g. `S.i()` with no args). Now returns `undefined` instead of crashing.
+
diff --git a/packages/compiler/src/lib/examples.ts b/packages/compiler/src/lib/examples.ts
index 9516fd638a6..90d61d0b278 100644
--- a/packages/compiler/src/lib/examples.ts
+++ b/packages/compiler/src/lib/examples.ts
@@ -229,7 +229,7 @@ function serializeScalarValueAsJson(
 
   const result = resolveKnownScalar(program, value.scalar);
   if (result === undefined) {
-    return serializeValueAsJson(program, value.value.args[0], value.value.args[0].type);
+    return undefined;
   }
 
   encodeAs = encodeAs ?? result.encodeAs;
diff --git a/packages/compiler/test/decorators/examples.test.ts b/packages/compiler/test/decorators/examples.test.ts
index 141e05655cf..b6d75bf81ec 100644
--- a/packages/compiler/test/decorators/examples.test.ts
+++ b/packages/compiler/test/decorators/examples.test.ts
@@ -130,6 +130,28 @@ describe("@example", () => {
         code: "unassignable",
       });
     });
+
+    it("returns undefined for custom scalar with no-argument initializer", async () => {
+      const { program, examples, target } = await getExamplesFor(`
+        @example(test.i())
+        @test scalar test {
+          init i();
+        }
+      `);
+      expect(examples).toHaveLength(1);
+      expect(serializeValueAsJson(program, examples[0].value, target)).toBeUndefined();
+    });
+
+    it("returns undefined for custom scalar with string-argument initializer", async () => {
+      const { program, examples, target } = await getExamplesFor(`
+        @example(test.name("Shorty"))
+        @test scalar test {
+          init name(value: string);
+        }
+      `);
+      expect(examples).toHaveLength(1);
+      expect(serializeValueAsJson(program, examples[0].value, target)).toBeUndefined();
+    });
   });
 
   describe("enum", () => {
diff --git a/packages/openapi3/test/models.test.ts b/packages/openapi3/test/models.test.ts
index 7ce314dec71..d88c53d8773 100644
--- a/packages/openapi3/test/models.test.ts
+++ b/packages/openapi3/test/models.test.ts
@@ -262,7 +262,7 @@ worksFor(supportedVersions, ({ diagnoseOpenApiFor, oapiForModel, openApiFor }) =
     });
   });
 
-  it("scalar used as a default value", async () => {
+  it("scalar with unknown constructor used as a default value produces no default and no diagnostic", async () => {
     const res = await oapiForModel(
       "Pet",
       `
@@ -272,7 +272,35 @@ worksFor(supportedVersions, ({ diagnoseOpenApiFor, oapiForModel, openApiFor }) =
       `,
     );
 
-    expect(res.schemas.Pet.properties.name.default).toEqual("Shorty");
+    expect(res.schemas.Pet.properties.name.default).toBeUndefined();
+  });
+
+  it("scalar with no-argument initializer used as a default value does not crash", async () => {
+    const res = await oapiForModel(
+      "M",
+      `
+        scalar S { init i(); }
+
+        model M { p: S = S.i(); }
+      `,
+    );
+
+    expect(res.schemas.M.properties.p.default).toBeUndefined();
+  });
+
+  it("known scalar constructors used as default values produce correct defaults", async () => {
+    const res = await oapiForModel(
+      "Foo",
+      `
+        model Foo {
+          int32Prop: int32 = int32(12);
+          stringProp: string = string("this is the string value");
+        }
+      `,
+    );
+
+    expect(res.schemas.Foo.properties.int32Prop.default).toEqual(12);
+    expect(res.schemas.Foo.properties.stringProp.default).toEqual("this is the string value");
   });
 
   it("encode know scalar as a default value", async () => {