diff --git a/etc/kaleidoscope-builder.conf b/etc/kaleidoscope-builder.conf
index 5032de7a..d66716aa 100644
--- a/etc/kaleidoscope-builder.conf
+++ b/etc/kaleidoscope-builder.conf
@@ -241,7 +241,7 @@ AR_BASENAME=$(basename ${AR:-ar})
 OBJCOPY_BASENAME=$(basename ${OBJCOPY:-objcopy})
 
 # Allow the compiler prefix to be empty for virtual builds
-: "${COMPILER_PREFIX:=avr-}"
+COMPILER_PREFIX="${COMPILER_PREFIX-avr-}"
 : "${AVR_SIZE:=${COMPILER_PATH}/${COMPILER_PREFIX}size}"
 : "${AVR_SIZE_FLAGS:=-C --mcu=${MCU}}"
 : "${AVR_OBJDUMP:=${COMPILER_PATH}/${COMPILER_PREFIX}objdump}"