diff --git a/configure.ac b/configure.ac
index 271471a7e8b3cd4f3474f03d0acbc05cb15779da..92d69f55139f768c576db154f1d0bb47d4247674 100644
--- a/configure.ac
+++ b/configure.ac
@@ -345,6 +345,8 @@ AC_C_INLINE
 AC_TYPE_UID_T
 AC_TYPE_SIZE_T
 AC_HEADER_TIME
+# Used by eratosthenes.c
+AC_CHECK_SIZEOF(long)
 
 AC_CHECK_HEADERS([openssl/blowfish.h openssl/des.h openssl/cast.h openssl/aes.h],,
 [enable_openssl=no