diff --git a/crcfactory.h b/crcfactory.h
index c006039..4825e1c 100644
--- a/crcfactory.h
+++ b/crcfactory.h
@@ -22,6 +22,7 @@
 #ifndef __CRCFACTORY_H__
 #define __CRCFACTORY_H__
 
+#include <limits.h>
 #include <stdint.h>
 #include <stddef.h>
 #include <stdbool.h>
@@ -101,10 +102,13 @@ CRCFACTORY_INLINE CRCFACTORY_CRC_TYPE _crcfactory_mask(int width, bool reflected
     if (!reflected && width <= 8) {
         return value & 0xff;
     }
-    // The following if statement is due to a bug under x86_64:
-    // "1ULL << 64" evaluates to 0 (expected), but
-    // "1ULL << x" when x==64 evaluates to 1 (wrong).
-    if ((1ULL << width) == 1) return value;
+
+    // Avoid undefined behavior if a shift to generate the mask would exceed
+    // the size of the type. Just return the value unmasked, since the mask
+    // would cover all bits anyway. Strictly speaking we probably want a
+    // limits-based check here in case CRC_TYPE is signed, but signed types
+    // would likely break other things anyway.
+    if ((size_t)width >= (CHAR_BIT * sizeof(CRCFACTORY_CRC_TYPE))) return value;
     return value & ((1ULL << width) - 1);
 }
 
diff --git a/test/crclist2test.py b/test/crclist2test.py
index 80ebb57..f5de642 100755
--- a/test/crclist2test.py
+++ b/test/crclist2test.py
@@ -6,6 +6,7 @@
 CFILE_TEMPLATE = r"""
 /* NOTE: This file was auto-generated by crclist2test.py */
 #include <stdio.h>
+#include <inttypes.h>
 
 #define CRCFACTORY_CRC_TYPE <<crc_datatype>>
 #define CRCFACTORY_CRCTABLE_TYPE <<table_datatype>>
@@ -20,7 +21,7 @@
     func##_t_init(crctable); \
     uint64_t r = func((uint8_t *)TESTDATA, TESTDATA_LEN); \
     uint64_t r_t = func##_t(crctable, (uint8_t *)TESTDATA, TESTDATA_LEN); \
-    printf("%-20s: %-16lx %-6s : %-16lx %-6s\n", desc, r, (r == check) ? "(OK)" : "(FAIL)", r_t, (r_t == check) ? "(OK)" : "(FAIL)"); \
+    printf("%-20s: %-16" PRIx64 " %-6s : %-16" PRIx64 " %-6s\n", desc, r, (r == check) ? "(OK)" : "(FAIL)", r_t, (r_t == check) ? "(OK)" : "(FAIL)"); \
     test_count += 2; \
     test_failures += (r == check) ? 0 : 1; \
     test_failures += (r_t == check) ? 0 : 1; \