diff --git a/include/mbedtls/gcm.h b/include/mbedtls/gcm.h
index 9d9155f..a4de919 100644
--- a/include/mbedtls/gcm.h
+++ b/include/mbedtls/gcm.h
@@ -45,6 +45,8 @@
 #define MBEDTLS_ERR_GCM_AUTH_FAILED                       -0x0012
 /** Bad input parameters to function. */
 #define MBEDTLS_ERR_GCM_BAD_INPUT                         -0x0014
+/** An output buffer is too small. */
+#define MBEDTLS_ERR_GCM_BUFFER_TOO_SMALL                  -0x0018
 
 #ifdef __cplusplus
 extern "C" {