diff --git a/docker_build.sh b/docker_build.sh
index 97ab5b6..1c0a8e1 100755
--- a/docker_build.sh
+++ b/docker_build.sh
@@ -1,8 +1,40 @@
 #!/bin/bash
 
-# update ENV vars to point to the first available file on disk
-export PBF2JSON_FILE=$(ls ${OSMPATH}/*.osm.pbf | head -n 1)
-export POLYLINE_FILE=$(ls ${POLYLINEPATH}/*.0sv | head -n 1)
+# ensure environment variables are set
+if [[ -z "${OSMPATH}" ]]; then
+  2>&1 echo "error: environment variable OSMPATH not set"
+  exit 1
+fi
+if [[ -z "${POLYLINEPATH}" ]]; then
+  2>&1 echo "error: environment variable POLYLINEPATH not set"
+  exit 1
+fi
+
+# read a list of available pbf/0sv files
+shopt -s nullglob
+PBF_FILES=(${OSMPATH}/*.pbf)
+POLYLINE_FILES=(${POLYLINEPATH}/*.0sv)
+shopt -u nullglob
+
+# ensure at least one *.pbf file exists, warn if more than one was found
+if [[ ${#PBF_FILES[@]} -eq 0 ]]; then
+  2>&1 echo "error: no .pbf files found in ${OSMPATH}"
+  exit 1
+elif [[ ${#PBF_FILES[@]} -gt 1 ]]; then
+  2>&1 echo "warning: multiple .pbf files found in ${OSMPATH}, only ${PBF_FILES[0]} was used"
+fi
+
+# ensure at least one *.0sv file exists, warn if more than one was found
+if [[ ${#POLYLINE_FILES[@]} -eq 0 ]]; then
+  2>&1 echo "error: no .0sv files found in ${POLYLINEPATH}"
+  exit 1
+elif [[ ${#POLYLINE_FILES[@]} -gt 1 ]]; then
+  2>&1 echo "warning: multiple .0sv files found in ${POLYLINEPATH}, only ${POLYLINE_FILES[0]} was used"
+fi
+
+# update ENV vars to point to the first available files on disk
+export PBF2JSON_FILE="${PBF_FILES[0]}"
+export POLYLINE_FILE="${POLYLINE_FILES[0]}"
 
 # run the build
 exec ./script/build.sh