diff --git a/flisp.h b/flisp.h
index 7eed78d..52ddbf9 100644
--- a/flisp.h
+++ b/flisp.h
@@ -6,6 +6,7 @@
 
 typedef uptrint_t value_t;
 typedef int_t fixnum_t;
+typedef uint_t ufixnum_t;
 #ifdef BITS64
 #define T_FIXNUM T_INT64
 #else
@@ -52,7 +53,7 @@ typedef struct {
 #define tag(x) ((x)&0x7)
 #define ptr(x) ((void *)((x) & (~(value_t)0x7)))
 #define tagptr(p, t) (((value_t)(p)) | (t))
-#define fixnum(x) ((value_t)(((fixnum_t)(x)) << 2))
+#define fixnum(x) ((value_t)(((ufixnum_t)(fixnum_t)(x)) << 2))
 #define numval(x) (((fixnum_t)(x)) >> 2)
 #ifdef BITS64
 #define fits_fixnum(x) (((x) >> 61) == 0 || (~((x) >> 61)) == 0)