diff --git a/ext/digest/bubblebabble/bubblebabble.c b/ext/digest/bubblebabble/bubblebabble.c
index dac603c..26c6081 100644
--- a/ext/digest/bubblebabble/bubblebabble.c
+++ b/ext/digest/bubblebabble/bubblebabble.c
@@ -14,6 +14,10 @@
 #include <ruby/ruby.h>
 #include "../digest.h"
 
+#ifndef HAVE_RB_LEN_T
+# define RB_LEN_MAX LONG_MAX
+#endif
+
 static ID id_digest;
 
 static VALUE
@@ -36,7 +40,7 @@ bubblebabble_str_new(VALUE str_digest)
     digest = RSTRING_PTR(str_digest);
     digest_len = RSTRING_LEN(str_digest);
 
-    if ((LONG_MAX - 2) / 3 < (digest_len | 1)) {
+    if ((RB_LEN_MAX - 2) / 3 < (digest_len | 1)) {
         rb_raise(rb_eRuntimeError, "digest string too long");
     }
 
diff --git a/ext/digest/digest.c b/ext/digest/digest.c
index c3feab1..9ae244f 100644
--- a/ext/digest/digest.c
+++ b/ext/digest/digest.c
@@ -17,6 +17,7 @@
 #include "digest.h"
 
 #ifndef HAVE_RB_LEN_T
+# define RB_LEN_MAX LONG_MAX
 # define LEN2NUM LONG2NUM
 #endif
 
@@ -116,7 +117,7 @@ hexencode_str_new(VALUE str_digest)
     digest = RSTRING_PTR(str_digest);
     digest_len = RSTRING_LEN(str_digest);
 
-    if (LONG_MAX / 2 < digest_len) {
+    if (RB_LEN_MAX / 2 < digest_len) {
         rb_raise(rb_eRuntimeError, "digest string too long");
     }