diff --git a/library/ctr_drbg.c b/library/ctr_drbg.c
index bc5cc8f63..64b536aaa 100644
--- a/library/ctr_drbg.c
+++ b/library/ctr_drbg.c
@@ -87,10 +87,6 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx )
     memset( ctx, 0, sizeof( mbedtls_ctr_drbg_context ) );
 
     ctx->reseed_interval = MBEDTLS_CTR_DRBG_RESEED_INTERVAL;
-
-#if defined(MBEDTLS_THREADING_C)
-    mbedtls_mutex_init( &ctx->mutex );
-#endif
 }
 
 /*
@@ -103,14 +99,12 @@ void mbedtls_ctr_drbg_free( mbedtls_ctr_drbg_context *ctx )
         return;
 
 #if defined(MBEDTLS_THREADING_C)
-    mbedtls_mutex_free( &ctx->mutex );
+    if( ctx->f_entropy != NULL )
+        mbedtls_mutex_free( &ctx->mutex );
 #endif
     mbedtls_aes_free( &ctx->aes_ctx );
     mbedtls_zeroize( ctx, sizeof( mbedtls_ctr_drbg_context ) );
     ctx->reseed_interval = MBEDTLS_CTR_DRBG_RESEED_INTERVAL;
-#if defined(MBEDTLS_THREADING_C)
-    mbedtls_mutex_init( &ctx->mutex );
-#endif
 }
 
 void mbedtls_ctr_drbg_set_prediction_resistance( mbedtls_ctr_drbg_context *ctx, int resistance )
@@ -382,6 +376,10 @@ int mbedtls_ctr_drbg_seed( mbedtls_ctr_drbg_context *ctx,
 
     memset( key, 0, MBEDTLS_CTR_DRBG_KEYSIZE );
 
+#if defined(MBEDTLS_THREADING_C)
+    mbedtls_mutex_init( &ctx->mutex );
+#endif
+
     mbedtls_aes_init( &ctx->aes_ctx );
 
     ctx->f_entropy = f_entropy;