diff --git a/library/lms.c b/library/lms.c
index 0c470a0..00525bb 100644
--- a/library/lms.c
+++ b/library/lms.c
@@ -505,7 +505,7 @@
     unsigned int height;
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
 
-    tree = mbedtls_calloc(MERKLE_TREE_NODE_AM(ctx->params.type),
+    tree = mbedtls_calloc((size_t) MERKLE_TREE_NODE_AM(ctx->params.type),
                           node_bytes);
     if (tree == NULL) {
         return MBEDTLS_ERR_LMS_ALLOC_FAILED;
@@ -530,7 +530,7 @@
 
 exit:
     mbedtls_zeroize_and_free(tree, node_bytes *
-                             MERKLE_TREE_NODE_AM(ctx->params.type));
+                             (size_t) MERKLE_TREE_NODE_AM(ctx->params.type));
 
     return ret;
 }
@@ -669,7 +669,7 @@
         return MBEDTLS_ERR_LMS_BAD_INPUT_DATA;
     }
 
-    tree = mbedtls_calloc(MERKLE_TREE_NODE_AM(priv_ctx->params.type),
+    tree = mbedtls_calloc((size_t) MERKLE_TREE_NODE_AM(priv_ctx->params.type),
                           node_bytes);
     if (tree == NULL) {
         return MBEDTLS_ERR_LMS_ALLOC_FAILED;
@@ -692,7 +692,7 @@
 
 exit:
     mbedtls_zeroize_and_free(tree, node_bytes *
-                             MERKLE_TREE_NODE_AM(priv_ctx->params.type));
+                             (size_t) MERKLE_TREE_NODE_AM(priv_ctx->params.type));
 
     return ret;
 }