diff --git a/openssl-sys/src/libressl.rs b/openssl-sys/src/libressl.rs
index 40490cd0..dc3bfe11 100644
--- a/openssl-sys/src/libressl.rs
+++ b/openssl-sys/src/libressl.rs
@@ -576,7 +576,7 @@ unsafe extern fn locking_function(mode: c_int, n: c_int, _file: *const c_char,
     if mode & ::CRYPTO_LOCK != 0 {
         (*GUARDS)[n as usize] = Some(mutex.lock().unwrap());
     } else {
-        &(*GUARDS)[n as usize].take();
+        &(*GUARDS)[n as usize].take().expect("lock already unlocked");
     }
 }
 
diff --git a/openssl-sys/src/ossl10x.rs b/openssl-sys/src/ossl10x.rs
index 0ffe3850..fade0f99 100644
--- a/openssl-sys/src/ossl10x.rs
+++ b/openssl-sys/src/ossl10x.rs
@@ -719,7 +719,7 @@ unsafe extern fn locking_function(mode: c_int, n: c_int, _file: *const c_char,
     if mode & ::CRYPTO_LOCK != 0 {
         (*GUARDS)[n as usize] = Some(mutex.lock().unwrap());
     } else {
-        &(*GUARDS)[n as usize].take();
+        &(*GUARDS)[n as usize].take().expect("lock already unlocked");
     }
 }