diff --git a/openssl-sys/src/lib.rs b/openssl-sys/src/lib.rs
index a482c357..649e269e 100755
--- a/openssl-sys/src/lib.rs
+++ b/openssl-sys/src/lib.rs
@@ -295,7 +295,7 @@ extern "C" {
                                                            n: c_int,
                                                            file: *const c_char,
                                                            line: c_int));
-    pub fn CRYPTO_free(buf: *const c_char);
+    pub fn CRYPTO_free(buf: *mut c_void);
     pub fn CRYPTO_memcmp(a: *const c_void, b: *const c_void,
                          len: size_t) -> c_int;
 
diff --git a/src/bn/mod.rs b/src/bn/mod.rs
index b33f94ce..2536f8a5 100644
--- a/src/bn/mod.rs
+++ b/src/bn/mod.rs
@@ -1,4 +1,4 @@
-use libc::{c_int, c_ulong};
+use libc::{c_int, c_ulong, c_void};
 use std::{fmt, ptr};
 use std::c_str::CString;
 
@@ -348,7 +348,7 @@ impl BigNum {
             assert!(!buf.is_null());
             let c_str = CString::new(buf, false);
             let str = c_str.as_str().unwrap().to_string();
-            ffi::CRYPTO_free(buf);
+            ffi::CRYPTO_free(buf as *mut c_void);
             str
         }
     }