diff --git a/tests/scripts/yotta-build.sh b/tests/scripts/yotta-build.sh
index 19cc576..4bae34a 100755
--- a/tests/scripts/yotta-build.sh
+++ b/tests/scripts/yotta-build.sh
@@ -1,12 +1,26 @@
 #!/bin/sh
 
-# Do test builds of the yotta module for all supported targets
+# yotta-build.sh
+#
+# This file is part of mbed TLS (https://tls.mbed.org)
+#
+# Copyright (c) 2015-2016, ARM Limited, All Rights Reserved
+#
+# Purpose
+#
+# To run test builds of the yotta module for all supported targets.
 
 set -eu
 
-yotta/create-module.sh
-cd yotta/module
-yt update || true # needs network
+check_tools()
+{
+    for TOOL in "$@"; do
+        if ! `hash "$TOOL" >/dev/null 2>&1`; then
+            echo "$TOOL not found!" >&2
+            exit 1
+        fi
+    done
+}
 
 yotta_build()
 {
@@ -19,22 +33,29 @@
     yt -t $TARGET build -d
 }
 
+# Make sure the tools we need are available.
+check_tools "arm-none-eabi-gcc" "armcc" "yotta"
+
+yotta/create-module.sh
+cd yotta/module
+yt update || true # needs network
+
 if uname -a | grep 'Linux.*x86' >/dev/null; then
     yotta_build x86-linux-native
 fi
 if uname -a | grep 'Darwin.*x86' >/dev/null; then
     yotta_build x86-osx-native
 fi
-if which armcc >/dev/null && armcc --help >/dev/null 2>&1; then
-    yotta_build frdm-k64f-armcc
-    #yotta_build nordic-nrf51822-16k-armcc
-fi
-if which arm-none-eabi-gcc >/dev/null; then
-    yotta_build frdm-k64f-gcc
-    #yotta_build st-nucleo-f401re-gcc # dirent
-    #yotta_build stm32f429i-disco-gcc # fails in mbed-hal-st-stm32f4
-    #yotta_build nordic-nrf51822-16k-gcc # fails in minar-platform
-    #yotta_build bbc-microbit-classic-gcc # fails in minar-platform
-    #yotta_build st-stm32f439zi-gcc # fails in mbed-hal-st-stm32f4
-    #yotta_build st-stm32f429i-disco-gcc # fails in mbed-hal-st-stm32f4
-fi
+
+# armcc build tests.
+yotta_build frdm-k64f-armcc
+#yotta_build nordic-nrf51822-16k-armcc
+
+# arm-none-eabi-gcc build tests.
+yotta_build frdm-k64f-gcc
+#yotta_build st-nucleo-f401re-gcc # dirent
+#yotta_build stm32f429i-disco-gcc # fails in mbed-hal-st-stm32f4
+#yotta_build nordic-nrf51822-16k-gcc # fails in minar-platform
+#yotta_build bbc-microbit-classic-gcc # fails in minar-platform
+#yotta_build st-stm32f439zi-gcc # fails in mbed-hal-st-stm32f4
+#yotta_build st-stm32f429i-disco-gcc # fails in mbed-hal-st-stm32f4