diff --git a/fantemp/terminal.hpp b/fantemp/terminal.hpp
index dd9e017..95bbeba 100644
--- a/fantemp/terminal.hpp
+++ b/fantemp/terminal.hpp
@@ -89,6 +89,8 @@ class Terminal {
 				printHelp();
 			} else if (strncmp_P(m_inputBuffer, reinterpret_cast<const char *>(SHOW_CMD), m_inputSize) == 0) {
 				showState();
+			} else {
+				printUnknown();
 			}
 		}
 	}
@@ -112,6 +114,14 @@ class Terminal {
 		m_serial << F("Temperature .: ") << floatBuffer << F(" C") << ENDL;
 		m_serial << F("Fan speed ...: ") << Controller::m_fanSpeed << F("%") << ENDL;
 	}
+
+	static void printUnknown()
+	{
+		m_serial << F("Unknown command \"");
+		for (uint16_t i = 0; i < m_inputSize; ++i)
+			m_serial << static_cast<char>(m_inputBuffer[i]);
+		m_serial << F("\"") << ENDL;
+	}
 };
 
 template <class Uart>