diff --git a/library/pem.c b/library/pem.c
index f090f49..a111970 100644
--- a/library/pem.c
+++ b/library/pem.c
@@ -244,6 +244,7 @@
 #if defined(MBEDTLS_DES_C) || defined(MBEDTLS_AES_C)
 static int pem_check_pkcs_padding(unsigned char *input, size_t input_len, size_t *data_len)
 {
+    /* input_len > 0 is guaranteed by mbedtls_pem_read_buffer(). */
     size_t pad_len = input[input_len - 1];
     size_t i;
 
@@ -412,6 +413,10 @@
         return MBEDTLS_ERROR_ADD(MBEDTLS_ERR_PEM_INVALID_DATA, ret);
     }
 
+    if (len == 0) {
+        return MBEDTLS_ERR_PEM_BAD_INPUT_DATA;
+    }
+
     if ((buf = mbedtls_calloc(1, len)) == NULL) {
         return MBEDTLS_ERR_PEM_ALLOC_FAILED;
     }