diff --git a/server/XfrmController.cpp b/server/XfrmController.cpp
index cc2c305..52ffd7b 100644
--- a/server/XfrmController.cpp
+++ b/server/XfrmController.cpp
@@ -146,7 +146,7 @@
     offset += 8;
 
     for (uint32_t j = 0; j < (uint32_t)len; j++) {
-        sprintf(&printBuf[j * 2 + offset], "%0.2x", buf[j]);
+        sprintf(&printBuf[j * 2 + offset], "%0.2x", (unsigned char)buf[j]);
     }
     ALOGD("%s", printBuf);
     delete[] printBuf;