diff --git a/libdevcore/CommonData.cpp b/libdevcore/CommonData.cpp
index 1299939f5..b151ba604 100644
--- a/libdevcore/CommonData.cpp
+++ b/libdevcore/CommonData.cpp
@@ -179,14 +179,13 @@ bool dev::isValidDecimal(string const& _string)
 	return true;
 }
 
-// Returns a quoted string if all characters are printable ASCII chars,
-// or its hex representation otherwise.
-std::string dev::formatAsStringOrNumber(std::string const& _value)
+string dev::formatAsStringOrNumber(string const& _value)
 {
-	if (_value.length() <= 32)
-		for (auto const& c: _value)
-			if (c <= 0x1f || c >= 0x7f || c == '"')
-				return "0x" + h256(_value, h256::AlignLeft).hex();
+	assertThrow(_value.length() <= 32, StringTooLong, "String to be formatted longer than 32 bytes.");
+
+	for (auto const& c: _value)
+		if (c <= 0x1f || c >= 0x7f || c == '"')
+			return "0x" + h256(_value, h256::AlignLeft).hex();
 
 	return "\"" + _value + "\"";
 }
diff --git a/libdevcore/CommonData.h b/libdevcore/CommonData.h
index 02b037ce8..9db1e3cf5 100644
--- a/libdevcore/CommonData.h
+++ b/libdevcore/CommonData.h
@@ -356,6 +356,7 @@ bool isValidDecimal(std::string const& _string);
 
 /// @returns a quoted string if all characters are printable ASCII chars,
 /// or its hex representation otherwise.
+/// _value cannot be longer than 32 bytes.
 std::string formatAsStringOrNumber(std::string const& _value);
 
 template<typename Container, typename Compare>
diff --git a/libdevcore/Exceptions.h b/libdevcore/Exceptions.h
index df7a9ba01..851be4ea1 100644
--- a/libdevcore/Exceptions.h
+++ b/libdevcore/Exceptions.h
@@ -49,6 +49,7 @@ DEV_SIMPLE_EXCEPTION(BadHexCharacter);
 DEV_SIMPLE_EXCEPTION(BadHexCase);
 DEV_SIMPLE_EXCEPTION(FileError);
 DEV_SIMPLE_EXCEPTION(DataTooLong);
+DEV_SIMPLE_EXCEPTION(StringTooLong);
 
 // error information to be added to exceptions
 using errinfo_comment = boost::error_info<struct tag_comment, std::string>;