diff --git a/src/gc.c b/src/gc.c
index 9e41abf14c709d4b026babe3eb319c65d26f01d4..a318ad88ce414b2b5cd4baf4e7904520ce7cb3c6 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -4580,7 +4580,9 @@ PMOD_EXPORT TYPE_FIELD real_visit_svalues (struct svalue *s, size_t num,
 
 #define MC_WQ_START_SIZE 1024
 
+#ifdef PIKE_THREADS
 static IMUTEX_T mc_mutex;
+#endif
 
 PMOD_EXPORT int mc_pass;
 PMOD_EXPORT size_t mc_counted_bytes;