diff --git a/arduino-mk/Arduino.mk b/arduino-mk/Arduino.mk
index e737abf..efa3bd6 100644
--- a/arduino-mk/Arduino.mk
+++ b/arduino-mk/Arduino.mk
@@ -257,7 +257,7 @@ ifndef ARDUINO_VERSION
     # Remove all the decimals, and right-pad with zeros, and finally grab the first 3 bytes.
     # Works for 1.0 and 1.0.1
     VERSION_FILE := $(ARDUINO_DIR)/lib/version.txt
-    AUTO_ARDUINO_VERSION := $(shell [ -e $(VERSION_FILE) ] && cat $(VERSION_FILE) | sed -e 's/[.]//g' -e 's/$$/0000/' | head -c3)
+    AUTO_ARDUINO_VERSION := $(shell [ -e $(VERSION_FILE) ] && cat $(VERSION_FILE) | sed -e 's/^[0-9]://g' -e 's/[.]//g' -e 's/$$/0000/' | head -c3)
     ifdef AUTO_ARDUINO_VERSION
         ARDUINO_VERSION = $(AUTO_ARDUINO_VERSION)
         $(call show_config_variable,ARDUINO_VERSION,[AUTODETECTED])