diff --git a/src/core/utils/parse_cmdline.cpp b/src/core/utils/parse_cmdline.cpp
index 8a500ff84..52f77dded 100644
--- a/src/core/utils/parse_cmdline.cpp
+++ b/src/core/utils/parse_cmdline.cpp
@@ -302,12 +302,17 @@ enum HexStringParseMode
 
 static Error ParseHexString(const char *&aString, uint16_t &aSize, uint8_t *aBuffer, HexStringParseMode aMode)
 {
-    Error  error        = kErrorNone;
-    size_t stringLength = strlen(aString);
-    size_t expectedSize = (stringLength + 1) / 2;
-    size_t parsedSize   = 0;
+    Error  error      = kErrorNone;
+    size_t parsedSize = 0;
+    size_t stringLength;
+    size_t expectedSize;
     bool   skipFirstDigit;
 
+    VerifyOrExit(aString != nullptr, error = kErrorInvalidArgs);
+
+    stringLength = strlen(aString);
+    expectedSize = (stringLength + 1) / 2;
+
     switch (aMode)
     {
     case kModeExtactSize: