diff --git a/firmware/libcommon/include/assert.h b/firmware/libcommon/include/assert.h
index 22bb101..dc6b1b5 100644
--- a/firmware/libcommon/include/assert.h
+++ b/firmware/libcommon/include/assert.h
@@ -89,7 +89,7 @@
 		/// \param condition  Condition to verify.
 		#define ASSERT(condition)  { \
 			if (!(condition)) { \
-				printf("-F- ASSERT: %s %s:%d\n\r", #condition, __BASE_FILE__, __LINE__); \
+				printf_sync("-F- ASSERT: %s %s:%d\n\r", #condition, __BASE_FILE__, __LINE__); \
 				while (1); \
 			} \
 		}
diff --git a/firmware/libosmocore/source/panic.c b/firmware/libosmocore/source/panic.c
index 74243b0..db5c26a 100644
--- a/firmware/libosmocore/source/panic.c
+++ b/firmware/libosmocore/source/panic.c
@@ -46,7 +46,7 @@
 __attribute__ ((format (printf, 1, 0)))
 static void osmo_panic_default(const char *fmt, va_list args)
 {
-	vfprintf(stderr, fmt, args);
+	vfprintf_sync(stderr, fmt, args);
 	osmo_generate_backtrace();
 	assert(0);
 }
diff --git a/firmware/test/card_emu_tests.c b/firmware/test/card_emu_tests.c
index 69159a8..5d5bd3e 100644
--- a/firmware/test/card_emu_tests.c
+++ b/firmware/test/card_emu_tests.c
@@ -13,6 +13,20 @@
 #define PHONE_INT	2
 #define PHONE_DATAOUT	3
 
+/* stub for stdio */
+signed int printf_sync(const char *pFormat, ...)
+{
+	va_list ap;
+	signed int result;
+
+	va_start(ap, pFormat);
+	result = vprintf(pFormat, ap);
+	va_end(ap);
+
+	return result;
+}
+
+
 /***********************************************************************
  * stub functions required by card_emu.c
  ***********************************************************************/