diff --git a/tests/unit/libutil-support/tests/string_callback.cc b/tests/unit/libutil-support/tests/string_callback.cc
index 2d0e0dad0..7a13bd4ff 100644
--- a/tests/unit/libutil-support/tests/string_callback.cc
+++ b/tests/unit/libutil-support/tests/string_callback.cc
@@ -2,9 +2,10 @@
 
 namespace nix::testing {
 
-void observe_string_cb(const char * start, unsigned int n, std::string * user_data)
+void observe_string_cb(const char * start, unsigned int n, void * user_data)
 {
-    *user_data = std::string(start);
+    auto user_data_casted = reinterpret_cast<std::string *>(user_data);
+    *user_data_casted = std::string(start);
 }
 
 }
diff --git a/tests/unit/libutil-support/tests/string_callback.hh b/tests/unit/libutil-support/tests/string_callback.hh
index a02ea3a1b..9a7e8d85d 100644
--- a/tests/unit/libutil-support/tests/string_callback.hh
+++ b/tests/unit/libutil-support/tests/string_callback.hh
@@ -3,14 +3,13 @@
 
 namespace nix::testing {
 
-void observe_string_cb(const char * start, unsigned int n, std::string * user_data);
+void observe_string_cb(const char * start, unsigned int n, void * user_data);
 
 inline void * observe_string_cb_data(std::string & out)
 {
     return (void *) &out;
 };
 
-#define OBSERVE_STRING(str) \
-    (nix_get_string_callback) nix::testing::observe_string_cb, nix::testing::observe_string_cb_data(str)
+#define OBSERVE_STRING(str) nix::testing::observe_string_cb, nix::testing::observe_string_cb_data(str)
 
 }