diff --git a/confutils.nim b/confutils.nim
index 19f5d5a..adca701 100644
--- a/confutils.nim
+++ b/confutils.nim
@@ -893,7 +893,7 @@ proc loadImpl[C, SecondarySources](
     quitOnFailure = true,
     secondarySourcesRef: ref SecondarySources,
     secondarySources: proc (config: Configuration,
-                            sources: ref SecondarySources) = nil,
+                            sources: ref SecondarySources) {.gcsafe.} = nil,
     envVarsPrefix = getAppFilename()): Configuration =
   ## Loads a program configuration by parsing command-line arguments
   ## and a standard set of config files that can specify:
diff --git a/tests/test_config_file.nim b/tests/test_config_file.nim
index c42575c..4422791 100644
--- a/tests/test_config_file.nim
+++ b/tests/test_config_file.nim
@@ -182,7 +182,11 @@ proc testConfigFile() =
     putEnv("PREFIX_DATA_DIR", "ENV VAR DATADIR")
 
     test "basic config file":
-      let conf = TestConf.load(envVarsPrefix="prefix", secondarySources = proc (config: TestConf, sources: auto) =
+      let conf = TestConf.load(
+          envVarsPrefix="prefix",
+          secondarySources = proc (
+            config: TestConf, sources: ref SecondarySources
+      ) {.raises: [ConfigurationError].} =
         if config.configFile.isSome:
           sources.addConfigFile(Toml, config.configFile.get)
         else: