diff --git a/src/libutil-c/nix_api_util.cc b/src/libutil-c/nix_api_util.cc
index c976b1815..cf468c8cd 100644
--- a/src/libutil-c/nix_api_util.cc
+++ b/src/libutil-c/nix_api_util.cc
@@ -112,7 +112,7 @@ const char * nix_err_msg(nix_c_context * context, const nix_c_context * read_con
 {
     if (context)
         context->last_err_code = NIX_OK;
-    if (read_context->last_err) {
+    if (read_context->last_err && read_context->last_err_code != NIX_OK) {
         if (n)
             *n = read_context->last_err->size();
         return read_context->last_err->c_str();
diff --git a/src/libutil-c/nix_api_util_internal.h b/src/libutil-c/nix_api_util_internal.h
index aa829feaf..7fa4252ac 100644
--- a/src/libutil-c/nix_api_util_internal.h
+++ b/src/libutil-c/nix_api_util_internal.h
@@ -10,6 +10,7 @@
 struct nix_c_context
 {
     nix_err last_err_code = NIX_OK;
+    /** The last error message. Always check last_err_code. This may not have been cleared, so that clearing is fast. */
     std::optional<std::string> last_err = {};
     std::optional<nix::ErrorInfo> info = {};
     std::string name = "";