diff --git a/kwset.c b/kwset.c
index 4fb6455aca..efc2ff41bc 100644
--- a/kwset.c
+++ b/kwset.c
@@ -38,7 +38,13 @@
 #include "compat/obstack.h"
 
 #define NCHAR (UCHAR_MAX + 1)
-#define obstack_chunk_alloc xmalloc
+/* adapter for `xmalloc()`, which takes `size_t`, not `long` */
+static void *obstack_chunk_alloc(long size)
+{
+	if (size < 0)
+		BUG("Cannot allocate a negative amount: %ld", size);
+	return xmalloc(size);
+}
 #define obstack_chunk_free free
 
 #define U(c) ((unsigned char) (c))