diff --git a/configure.in b/configure.in
index c81709e6..5618b3a2 100644
--- a/configure.in
+++ b/configure.in
@@ -247,6 +247,10 @@ AC_TRY_COMPILE([#include <sys/types.h>
 AC_C_BIGENDIAN([AC_DEFINE(CPU_BIG_ENDIAN)], [AC_DEFINE(CPU_LITTLE_ENDIAN)],
 		 [AC_MSG_ERROR([Cannot determine CPU endianity.])])
 
+AC_CHECK_SIZEOF(unsigned int)
+AC_CHECK_SIZEOF(unsigned long)
+AC_CHECK_SIZEOF(unsigned long long)
+
 BIRD_CHECK_INTEGERS
 BIRD_CHECK_STRUCT_ALIGN
 BIRD_CHECK_TIME_T
diff --git a/lib/sha512.c b/lib/sha512.c
index becb91b9..88343875 100644
--- a/lib/sha512.c
+++ b/lib/sha512.c
@@ -20,7 +20,13 @@
 #include "lib/sha512.h"
 #include "lib/unaligned.h"
 
-#define U64_C(c) (c ## UL) /* Maybe is system dependent */
+#if SIZEOF_UNSIGNED_INT == 8
+  #define U64_C(c) (c ## U)
+#elif SIZEOF_UNSIGNED_LONG == 8
+  #define U64_C(c) (c ## UL)
+#elif SIZEOF_UNSIGNED_LONG_LONG == 8
+  #define U64_C(c) (c ## ULL)
+#endif
 
 static uint sha512_transform(void *context, const byte *data, size_t nblks);
 
diff --git a/sysdep/autoconf.h.in b/sysdep/autoconf.h.in
index a9e46e27..25729849 100644
--- a/sysdep/autoconf.h.in
+++ b/sysdep/autoconf.h.in
@@ -66,4 +66,13 @@
 /* We have stdint.h */
 #undef HAVE_STDINT_H
 
+/* The size of `unsigned int', as computed by sizeof. */
+#undef SIZEOF_UNSIGNED_INT
+
+/* The size of `unsigned long', as computed by sizeof. */
+#undef SIZEOF_UNSIGNED_LONG
+
+/* The size of `unsigned long long', as computed by sizeof. */
+#undef SIZEOF_UNSIGNED_LONG_LONG
+
 #define CONFIG_PATH ?
diff --git a/sysdep/config.h b/sysdep/config.h
index 08c15fe9..800118e9 100644
--- a/sysdep/config.h
+++ b/sysdep/config.h
@@ -15,6 +15,18 @@
 /* Include OS configuration file as chosen in autoconf.h */
 #include SYSCONF_INCLUDE
 
+/* The AC_CHECK_SIZEOF() in configure fails for some machines.
+ * we provide some fallback values here */
+#ifndef SIZEOF_UNSIGNED_INT
+# define SIZEOF_UNSIGNED_INT 4
+#endif
+#ifndef SIZEOF_UNSIGNED_LONG
+# define SIZEOF_UNSIGNED_LONG 4
+#endif
+#ifndef SIZEOF_UNSIGNED_LONG_LONG
+# define SIZEOF_UNSIGNED_LONG_LONG 8
+#endif
+
 #ifndef MACROS_ONLY
 
 /*