diff --git a/tslang/lib/TypeScript/MLIRGenImpl.h b/tslang/lib/TypeScript/MLIRGenImpl.h
index b62a7b92e..6cfa65f62 100644
--- a/tslang/lib/TypeScript/MLIRGenImpl.h
+++ b/tslang/lib/TypeScript/MLIRGenImpl.h
@@ -5634,6 +5634,18 @@ class MLIRGenImpl
             return mlir::failure();
         }
 
+        // a number, a string, a boolean or a bigint is no class to test against. A type of one
+        // (`type str = TypeOf<"">`) is: the test compares the types (tests/00instanceof.ts).
+        auto rightBaseType = mth.getBaseType(rightType);
+        if (!resultRightValue.getDefiningOp<mlir_ts::TypeRefOp>()
+            && (mth.isNumericType(rightBaseType) || isa<mlir_ts::StringType>(rightBaseType)
+                || isa<mlir_ts::BooleanType>(rightBaseType) || isa<mlir_ts::BigIntType>(rightBaseType)))
+        {
+            emitError(loc(binaryExpressionAST->right), "The right-hand side of an 'instanceof' expression must be a class, not '")
+                << to_print(rightType) << "'";
+            return mlir::failure();
+        }
+
         rightType = mth.wideStorageType(rightType);
 
         if (isCppVTableClass(rightType) || isCppVTableClass(resultLeftfType))
diff --git a/tslang/test/tester/CMakeLists.txt b/tslang/test/tester/CMakeLists.txt
index e92db2eb9..d98030733 100644
--- a/tslang/test/tester/CMakeLists.txt
+++ b/tslang/test/tester/CMakeLists.txt
@@ -2748,6 +2748,16 @@ add_test(NAME test-compile-interface-cast-empty
 set_tests_properties(test-compile-interface-cast-empty
                      PROPERTIES FAIL_REGULAR_EXPRESSION "error:|Stack dump|Assertion failed")
 
+# `instanceof` with a number, a string, a boolean or a bigint value on the right is an error: the
+# test compared the two types. A type there (`type str = TypeOf<"">`) stays an extension.
+add_test(NAME test-compile-diagnostic-location-instanceof-primitive
+         COMMAND $<TARGET_FILE:tslang> --emit=obj --no-default-lib -mm=none
+                 "${PROJECT_SOURCE_DIR}/test/tester/diagnostic-location/instanceof_primitive.ts"
+                 -o "${CMAKE_CURRENT_BINARY_DIR}/diagnostic-location-instanceof-primitive.obj")
+set_tests_properties(test-compile-diagnostic-location-instanceof-primitive
+                     PROPERTIES PASS_REGULAR_EXPRESSION "instanceof_primitive\\.ts:8:24: error: The right-hand side of an 'instanceof' expression must be a class, not '3'"
+                                FAIL_REGULAR_EXPRESSION "Stack dump|Assertion failed|instanceof_primitive\\.ts:7:")
+
 # A @cpp class names the slots of a vtable made by C++ (tests/00cppvtable_class.ts), so what
 # would add a slot, run code of its own or ask the object what it is is an error: a slot off by one
 # would call the wrong C++ method, with no error at all. One whose methods have bodies implements
diff --git a/tslang/test/tester/diagnostic-location/instanceof_primitive.ts b/tslang/test/tester/diagnostic-location/instanceof_primitive.ts
new file mode 100644
index 000000000..e8b9e6465
--- /dev/null
+++ b/tslang/test/tester/diagnostic-location/instanceof_primitive.ts
@@ -0,0 +1,9 @@
+// A number on the right of `instanceof` is no class: the test compared the two types and
+// was quietly true or false.
+
+type str = TypeOf<"">;
+
+function main() {
+    print("asd" instanceof str);
+    print(5 instanceof 3);
+}