diff --git a/pw_function/function_test.cc b/pw_function/function_test.cc
index 2f25cfc7df..b7309c3169 100644
--- a/pw_function/function_test.cc
+++ b/pw_function/function_test.cc
@@ -22,24 +22,24 @@ namespace pw {
 namespace {
 
 #if PW_NC_TEST(CannotInstantiateWithNonFunction)
-PW_NC_EXPECT("incomplete type|undefined template");
+PW_NC_EXPECT("must be instantiated with a function type");
 
 [[maybe_unused]] Function<int> function_pointer;
 
 #elif PW_NC_TEST(CannotInstantiateWithFunctionPointer1)
-PW_NC_EXPECT("incomplete type|undefined template");
+PW_NC_EXPECT("must be instantiated with a function type");
 
 [[maybe_unused]] Function<void (*)()> function_pointer;
 
 #elif PW_NC_TEST(CannotInstantiateWithFunctionPointer2)
-PW_NC_EXPECT("incomplete type|undefined template");
+PW_NC_EXPECT("must be instantiated with a function type");
 
 [[maybe_unused]] void SomeFunction(int);
 
 [[maybe_unused]] Function<decltype(&SomeFunction)> function_pointer;
 
 #elif PW_NC_TEST(CannotInstantiateWithFunctionReference)
-PW_NC_EXPECT("incomplete type|undefined template");
+PW_NC_EXPECT("must be instantiated with a function type");
 
 [[maybe_unused]] Function<void (&)()> function_pointer;
 
diff --git a/third_party/fuchsia/repo/sdk/lib/fit/include/lib/fit/function.h b/third_party/fuchsia/repo/sdk/lib/fit/include/lib/fit/function.h
index e11970550d..cc7ff00cf8 100644
--- a/third_party/fuchsia/repo/sdk/lib/fit/include/lib/fit/function.h
+++ b/third_party/fuchsia/repo/sdk/lib/fit/include/lib/fit/function.h
@@ -5,17 +5,27 @@
 #ifndef LIB_FIT_INCLUDE_LIB_FIT_FUNCTION_H_
 #define LIB_FIT_INCLUDE_LIB_FIT_FUNCTION_H_
 
+#include <type_traits>
+
 #include "function_internal.h"
 #include "traits.h"
 #include "utility_internal.h"
 
 namespace fit {
 
-template <size_t inline_target_size, bool require_inline, typename Result, typename... Args>
-class function_impl;
+template <size_t inline_target_size, bool require_inline, typename Callable>
+class function_impl {
+  static_assert(std::is_function<Callable>::value,
+                "fit::function must be instantiated with a function type, such as void() or "
+                "int(char*, bool)");
+};
 
-template <size_t inline_target_size, bool require_inline, typename Result, typename... Args>
-class callback_impl;
+template <size_t inline_target_size, bool require_inline, typename Callable>
+class callback_impl {
+  static_assert(std::is_function<Callable>::value,
+                "fit::callback must be instantiated with a function type, such as void() or "
+                "int(char*, bool)");
+};
 
 // The default size allowance for storing a target inline within a function
 // object, in bytes.  This default allows for inline storage of targets
@@ -302,30 +312,30 @@ class function_impl<inline_target_size, require_inline, Result(Args...)> final
   }
 };
 
-template <size_t inline_target_size, bool require_inline, typename Result, typename... Args>
-void swap(function_impl<inline_target_size, require_inline, Result, Args...>& a,
-          function_impl<inline_target_size, require_inline, Result, Args...>& b) {
+template <size_t inline_target_size, bool require_inline, typename Callable>
+void swap(function_impl<inline_target_size, require_inline, Callable>& a,
+          function_impl<inline_target_size, require_inline, Callable>& b) {
   a.swap(b);
 }
 
-template <size_t inline_target_size, bool require_inline, typename Result, typename... Args>
-bool operator==(const function_impl<inline_target_size, require_inline, Result, Args...>& f,
+template <size_t inline_target_size, bool require_inline, typename Callable>
+bool operator==(const function_impl<inline_target_size, require_inline, Callable>& f,
                 decltype(nullptr)) {
   return !f;
 }
-template <size_t inline_target_size, bool require_inline, typename Result, typename... Args>
+template <size_t inline_target_size, bool require_inline, typename Callable>
 bool operator==(decltype(nullptr),
-                const function_impl<inline_target_size, require_inline, Result, Args...>& f) {
+                const function_impl<inline_target_size, require_inline, Callable>& f) {
   return !f;
 }
-template <size_t inline_target_size, bool require_inline, typename Result, typename... Args>
-bool operator!=(const function_impl<inline_target_size, require_inline, Result, Args...>& f,
+template <size_t inline_target_size, bool require_inline, typename Callable>
+bool operator!=(const function_impl<inline_target_size, require_inline, Callable>& f,
                 decltype(nullptr)) {
   return !!f;
 }
-template <size_t inline_target_size, bool require_inline, typename Result, typename... Args>
+template <size_t inline_target_size, bool require_inline, typename Callable>
 bool operator!=(decltype(nullptr),
-                const function_impl<inline_target_size, require_inline, Result, Args...>& f) {
+                const function_impl<inline_target_size, require_inline, Callable>& f) {
   return !!f;
 }
 
@@ -458,30 +468,30 @@ class callback_impl<inline_target_size, require_inline, Result(Args...)> final
   }
 };
 
-template <size_t inline_target_size, bool require_inline, typename Result, typename... Args>
-void swap(callback_impl<inline_target_size, require_inline, Result, Args...>& a,
-          callback_impl<inline_target_size, require_inline, Result, Args...>& b) {
+template <size_t inline_target_size, bool require_inline, typename Callable>
+void swap(callback_impl<inline_target_size, require_inline, Callable>& a,
+          callback_impl<inline_target_size, require_inline, Callable>& b) {
   a.swap(b);
 }
 
-template <size_t inline_target_size, bool require_inline, typename Result, typename... Args>
-bool operator==(const callback_impl<inline_target_size, require_inline, Result, Args...>& f,
+template <size_t inline_target_size, bool require_inline, typename Callable>
+bool operator==(const callback_impl<inline_target_size, require_inline, Callable>& f,
                 decltype(nullptr)) {
   return !f;
 }
-template <size_t inline_target_size, bool require_inline, typename Result, typename... Args>
+template <size_t inline_target_size, bool require_inline, typename Callable>
 bool operator==(decltype(nullptr),
-                const callback_impl<inline_target_size, require_inline, Result, Args...>& f) {
+                const callback_impl<inline_target_size, require_inline, Callable>& f) {
   return !f;
 }
-template <size_t inline_target_size, bool require_inline, typename Result, typename... Args>
-bool operator!=(const callback_impl<inline_target_size, require_inline, Result, Args...>& f,
+template <size_t inline_target_size, bool require_inline, typename Callable>
+bool operator!=(const callback_impl<inline_target_size, require_inline, Callable>& f,
                 decltype(nullptr)) {
   return !!f;
 }
-template <size_t inline_target_size, bool require_inline, typename Result, typename... Args>
+template <size_t inline_target_size, bool require_inline, typename Callable>
 bool operator!=(decltype(nullptr),
-                const callback_impl<inline_target_size, require_inline, Result, Args...>& f) {
+                const callback_impl<inline_target_size, require_inline, Callable>& f) {
   return !!f;
 }
 
diff --git a/third_party/fuchsia/repo/sdk/lib/fit/include/lib/fit/function_internal.h b/third_party/fuchsia/repo/sdk/lib/fit/include/lib/fit/function_internal.h
index 649373c8e1..0e226e99d4 100644
--- a/third_party/fuchsia/repo/sdk/lib/fit/include/lib/fit/function_internal.h
+++ b/third_party/fuchsia/repo/sdk/lib/fit/include/lib/fit/function_internal.h
@@ -179,7 +179,7 @@ constexpr target_ops<Result, Args...> target<SharedFunction,
                                              /*is_shared=*/true, Result, Args...>::ops = {
     &target::target_type_id, &target::get, &target::invoke, &target::move, &target::destroy};
 
-template <size_t inline_target_size, bool require_inline, typename Result, typename... Args>
+template <size_t inline_target_size, bool require_inline, typename Callable>
 class function_base;
 
 // Function implementation details.