diff --git a/gc_heap.c b/gc_heap.c
index 0b47505a..b91c5281 100644
--- a/gc_heap.c
+++ b/gc_heap.c
@@ -635,7 +635,7 @@ sexp sexp_load_image (const char* filename, off_t offset, sexp_uint_t heap_free_
     goto done;
   }
   if (offset > 0 && fseek(fp, offset, SEEK_SET) < 0) {
-    snprintf(gc_heap_err_str, ERR_STR_SIZE, "couldn't seek to image offset: %s -> "PRIoff": %s\n", filename, offset, strerror(errno));
+    snprintf(gc_heap_err_str, ERR_STR_SIZE, "couldn't seek to image offset: %s -> %"SEXP_PRIdOFF": %s\n", filename, offset, strerror(errno));
     goto done;
   }
 
diff --git a/include/chibi/sexp.h b/include/chibi/sexp.h
index 1a97c683..151427f1 100644
--- a/include/chibi/sexp.h
+++ b/include/chibi/sexp.h
@@ -283,9 +283,20 @@ typedef short sexp_int32_t;
 #endif
 
 #if defined(__APPLE__) || defined(_WIN64) || (defined(__CYGWIN__) && __SIZEOF_POINTER__ == 8)
-#define PRIoff "%lld"
+#define SEXP_PRIdOFF "lld"
 #else
-#define PRIoff "%ld"
+#define SEXP_PRIdOFF "ld"
+#endif
+
+#if defined(__GNUC__) || defined(_WIN64) || defined(__APPLE__)
+#include <stdint.h>
+#if SEXP_64_BIT
+#define SEXP_PRIdFIXNUM PRId64
+#else
+#define SEXP_PRIdFIXNUM PRId32
+#endif
+#else
+#define SEXP_PRIdFIXNUM "ld"
 #endif
 
 #if SEXP_USE_LONG_PROCEDURE_ARGS
diff --git a/sexp.c b/sexp.c
index 68bc6235..d4d2b998 100644
--- a/sexp.c
+++ b/sexp.c
@@ -2111,7 +2111,7 @@ sexp sexp_write_one (sexp ctx, sexp obj, sexp out) {
       break;
     }
   } else if (sexp_fixnump(obj)) {
-    snprintf(numbuf, NUMBUF_LEN, "%ld", (long)sexp_unbox_fixnum(obj));
+    snprintf(numbuf, NUMBUF_LEN, "%" SEXP_PRIdFIXNUM, (long)sexp_unbox_fixnum(obj));
     sexp_write_string(ctx, numbuf, out);
 #if SEXP_USE_IMMEDIATE_FLONUMS
   } else if (sexp_flonump(obj)) {