diff --git a/library/hmac_drbg.c b/library/hmac_drbg.c
index 25a0225..d322b65 100644
--- a/library/hmac_drbg.c
+++ b/library/hmac_drbg.c
@@ -54,10 +54,6 @@
     memset( ctx, 0, sizeof( mbedtls_hmac_drbg_context ) );
 
     ctx->reseed_interval = MBEDTLS_HMAC_DRBG_RESEED_INTERVAL;
-
-#if defined(MBEDTLS_THREADING_C)
-    mbedtls_mutex_init( &ctx->mutex );
-#endif
 }
 
 /*
@@ -129,6 +125,10 @@
     if( ( ret = mbedtls_md_setup( &ctx->md_ctx, md_info, 1 ) ) != 0 )
         return( ret );
 
+#if defined(MBEDTLS_THREADING_C)
+    mbedtls_mutex_init( &ctx->mutex );
+#endif
+
     /*
      * Set initial working state.
      * Use the V memory location, which is currently all 0, to initialize the
@@ -254,6 +254,10 @@
     if( ( ret = mbedtls_md_setup( &ctx->md_ctx, md_info, 1 ) ) != 0 )
         return( ret );
 
+#if defined(MBEDTLS_THREADING_C)
+    mbedtls_mutex_init( &ctx->mutex );
+#endif
+
     md_size = mbedtls_md_get_size( md_info );
 
     /*
@@ -421,14 +425,12 @@
         return;
 
 #if defined(MBEDTLS_THREADING_C)
-    mbedtls_mutex_free( &ctx->mutex );
+    if( ctx->md_ctx.md_info != NULL )
+        mbedtls_mutex_free( &ctx->mutex );
 #endif
     mbedtls_md_free( &ctx->md_ctx );
     mbedtls_platform_zeroize( ctx, sizeof( mbedtls_hmac_drbg_context ) );
     ctx->reseed_interval = MBEDTLS_HMAC_DRBG_RESEED_INTERVAL;
-#if defined(MBEDTLS_THREADING_C)
-    mbedtls_mutex_init( &ctx->mutex );
-#endif
 }
 
 #if defined(MBEDTLS_FS_IO)