diff --git a/src/pkg/runtime/mheapmap32.c b/src/pkg/runtime/mheapmap32.c
index 13491595da..1e3598cbea 100644
--- a/src/pkg/runtime/mheapmap32.c
+++ b/src/pkg/runtime/mheapmap32.c
@@ -10,7 +10,7 @@
 
 // 3-level radix tree mapping page ids to Span*.
 void
-MHeapMap_Init(MHeapMap *m, void *(*allocator)(size_t))
+MHeapMap_Init(MHeapMap *m, void *(*allocator)(uintptr))
 {
 	m->allocator = allocator;
 }
diff --git a/src/pkg/runtime/mheapmap64.c b/src/pkg/runtime/mheapmap64.c
index 97e20b6d20..2f856ee179 100644
--- a/src/pkg/runtime/mheapmap64.c
+++ b/src/pkg/runtime/mheapmap64.c
@@ -10,7 +10,7 @@
 
 // 3-level radix tree mapping page ids to Span*.
 void
-MHeapMap_Init(MHeapMap *m, void *(*allocator)(size_t))
+MHeapMap_Init(MHeapMap *m, void *(*allocator)(uintptr))
 {
 	m->allocator = allocator;
 }