diff --git a/cache.h b/cache.h
index 5c035dae57..e7b0e6be95 100644
--- a/cache.h
+++ b/cache.h
@@ -1790,8 +1790,11 @@ extern int git_config_include(const char *name, const char *value, void *data);
  *
  * (i.e., what gets handed to a config_fn_t). The caller provides the section;
  * we return -1 if it does not match, 0 otherwise. The subsection and key
- * out-parameters are filled by the function (and subsection is NULL if it is
+ * out-parameters are filled by the function (and *subsection is NULL if it is
  * missing).
+ *
+ * If the subsection pointer-to-pointer passed in is NULL, returns 0 only if
+ * there is no subsection at all.
  */
 extern int parse_config_key(const char *var,
 			    const char *section,
diff --git a/config.c b/config.c
index a23f260792..fd92738fba 100644
--- a/config.c
+++ b/config.c
@@ -2547,10 +2547,14 @@ int parse_config_key(const char *var,
 
 	/* Did we have a subsection at all? */
 	if (dot == var) {
-		*subsection = NULL;
-		*subsection_len = 0;
+		if (subsection) {
+			*subsection = NULL;
+			*subsection_len = 0;
+		}
 	}
 	else {
+		if (!subsection)
+			return -1;
 		*subsection = var + 1;
 		*subsection_len = dot - *subsection;
 	}