diff --git a/source/fuzz/fuzzer_pass_replace_irrelevant_ids.cpp b/source/fuzz/fuzzer_pass_replace_irrelevant_ids.cpp
index 71f3507..f35ee50 100644
--- a/source/fuzz/fuzzer_pass_replace_irrelevant_ids.cpp
+++ b/source/fuzz/fuzzer_pass_replace_irrelevant_ids.cpp
@@ -70,10 +70,12 @@
   }
 
   // For every type for which we have at least one irrelevant id, record all ids
-  // in the module which have that type.
+  // in the module which have that type. Skip ids of OpFunction instructions as
+  // we cannot use these as replacements.
   for (const auto& pair : GetIRContext()->get_def_use_mgr()->id_to_defs()) {
     uint32_t type_id = pair.second->type_id();
-    if (type_id && types_to_ids.count(type_id)) {
+    if (pair.second->opcode() != SpvOpFunction && type_id &&
+        types_to_ids.count(type_id)) {
       types_to_ids[type_id].push_back(pair.first);
     }
   }
@@ -122,7 +124,7 @@
           std::vector<uint32_t> available_replacement_ids;
 
           for (auto replacement_id : types_to_ids[type_id]) {
-            // We cannot replace an id with itself.
+            // It would be pointless to replace an id with itself.
             if (replacement_id == irrelevant_id) {
               continue;
             }
diff --git a/source/fuzz/transformation_replace_irrelevant_id.cpp b/source/fuzz/transformation_replace_irrelevant_id.cpp
index 7cd4eff..5cea04d 100644
--- a/source/fuzz/transformation_replace_irrelevant_id.cpp
+++ b/source/fuzz/transformation_replace_irrelevant_id.cpp
@@ -64,6 +64,11 @@
     return false;
   }
 
+  // The replacement id must not be the result of an OpFunction instruction.
+  if (replacement_id_def->opcode() == SpvOpFunction) {
+    return false;
+  }
+
   // Consistency check: an irrelevant id cannot be a pointer.
   assert(
       !ir_context->get_type_mgr()->GetType(type_id_of_interest)->AsPointer() &&
diff --git a/source/fuzz/transformation_replace_irrelevant_id.h b/source/fuzz/transformation_replace_irrelevant_id.h
index 0210520..35b1987 100644
--- a/source/fuzz/transformation_replace_irrelevant_id.h
+++ b/source/fuzz/transformation_replace_irrelevant_id.h
@@ -32,6 +32,7 @@
   // - The id of interest in |message_.id_use_descriptor| is irrelevant
   //   according to the fact manager.
   // - The types of the original id and of the replacement ids are the same.
+  // - The replacement must not be the result id of an OpFunction instruction.
   // - |message_.replacement_id| is available to use at the enclosing
   //   instruction of |message_.id_use_descriptor|.
   // - The original id is in principle replaceable with any other id of the same
diff --git a/test/fuzz/transformation_replace_irrelevant_id_test.cpp b/test/fuzz/transformation_replace_irrelevant_id_test.cpp
index 345ef86..828fd82 100644
--- a/test/fuzz/transformation_replace_irrelevant_id_test.cpp
+++ b/test/fuzz/transformation_replace_irrelevant_id_test.cpp
@@ -236,6 +236,55 @@
                    .IsApplicable(context.get(), transformation_context));
 }
 
+TEST(TransformationReplaceIrrelevantIdTest,
+     DoNotReplaceIrrelevantIdWithOpFunction) {
+  // Checks that an OpFunction result id is not allowed to be used to replace an
+  // irrelevant id.
+  const std::string reference_shader = R"(
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %4 "main"
+               OpExecutionMode %4 OriginUpperLeft
+               OpSource ESSL 320
+          %2 = OpTypeVoid
+          %3 = OpTypeFunction %2
+          %6 = OpTypeInt 32 1
+          %7 = OpTypeFunction %6
+         %13 = OpConstant %6 2
+          %4 = OpFunction %2 None %3
+          %5 = OpLabel
+         %20 = OpCopyObject %6 %13
+         %21 = OpCopyObject %6 %20
+               OpReturn
+               OpFunctionEnd
+         %10 = OpFunction %6 None %7
+         %11 = OpLabel
+               OpReturnValue %13
+               OpFunctionEnd
+  )";
+
+  const auto env = SPV_ENV_UNIVERSAL_1_5;
+  const auto consumer = nullptr;
+  const auto context =
+      BuildModule(env, consumer, reference_shader, kFuzzAssembleOption);
+  spvtools::ValidatorOptions validator_options;
+  ASSERT_TRUE(fuzzerutil::IsValidAndWellFormed(context.get(), validator_options,
+                                               kConsoleMessageConsumer));
+  TransformationContext transformation_context(
+      MakeUnique<FactManager>(context.get()), validator_options);
+  transformation_context.GetFactManager()->AddFactIdIsIrrelevant(20);
+
+  // We cannot replace the use of %20 in by %21 with %10 because %10 is an
+  // OpFunction instruction.
+  ASSERT_FALSE(
+      TransformationReplaceIrrelevantId(
+          MakeIdUseDescriptor(
+              20, MakeInstructionDescriptor(21, SpvOpCopyObject, 0), 0),
+          10)
+          .IsApplicable(context.get(), transformation_context));
+}
+
 }  // namespace
 }  // namespace fuzz
 }  // namespace spvtools