diff --git a/src/libexpr/eval-gc.hh b/src/libexpr/eval-gc.hh
index 76f019729..005175eb7 100644
--- a/src/libexpr/eval-gc.hh
+++ b/src/libexpr/eval-gc.hh
@@ -15,9 +15,11 @@ void initGC();
  */
 void assertGCInitialized();
 
+#ifdef HAVE_BOEHMGC
 /**
  * The number of GC cycles since initGC().
  */
 size_t getGCCycles();
+#endif
 
-}
+} // namespace nix