diff --git a/library/lms.c b/library/lms.c
index 26c48ef..f1230c7 100644
--- a/library/lms.c
+++ b/library/lms.c
@@ -562,7 +562,7 @@
            ctx->params.I_key_identifier,
            MBEDTLS_LMOTS_I_KEY_ID_LEN );
 
-    ctx->ots_private_keys = mbedtls_calloc( MERKLE_TREE_LEAF_NODE_AM(ctx->params.type),
+    ctx->ots_private_keys = mbedtls_calloc( ( size_t )MERKLE_TREE_LEAF_NODE_AM(ctx->params.type),
                                             sizeof( mbedtls_lmots_private_t ) );
     if( ctx->ots_private_keys == NULL )
     {
@@ -570,7 +570,7 @@
         goto exit;
     }
 
-    ctx->ots_public_keys = mbedtls_calloc( MERKLE_TREE_LEAF_NODE_AM(ctx->params.type),
+    ctx->ots_public_keys = mbedtls_calloc( ( size_t )MERKLE_TREE_LEAF_NODE_AM(ctx->params.type),
                                            sizeof( mbedtls_lmots_public_t ) );
     if( ctx->ots_public_keys == NULL )
     {