diff --git a/src/importer/index.ts b/src/importer/index.ts
index 5874c2b..a2bf4be 100644
--- a/src/importer/index.ts
+++ b/src/importer/index.ts
@@ -3,6 +3,14 @@ import { adapters } from "../adapters";
 import { DummyOpenHAB, OpenHAB } from "../openhab";
 
 
+const createFilter = (filter?: string) => {
+  if(!filter) {
+    return (_: Thing) => true;
+  }
+  
+  return new Function("$", `const v = ${filter}; if(typeof v !== 'boolean') throw new Error("Filter should be of boolean type"); return v;`) as ((_: Thing) => true);
+};
+
 export const importThings = async (config: Config, options: ImportOptions) => {  
   const OH = options.dryRun ? DummyOpenHAB : OpenHAB;
   const openhab = new OH(config.openHAB);
@@ -24,9 +32,7 @@ export const importThings = async (config: Config, options: ImportOptions) => {
   const existingThings = await openhab.getThings();
   const existingThingsUIDs = existingThings.map(t => t.UID);  
 
-  const filter: (thing: Thing) => boolean = options.where 
-    ? (new Function("$", `return ${options.where}`)) as ((_: Thing) => true)
-    : (_: Thing) => true;
+  const filter = createFilter(options.where);
   
   const thingsToImport = things
     .filter(t => !existingThingsUIDs.includes(t.UID))
@@ -39,4 +45,4 @@ export const importThings = async (config: Config, options: ImportOptions) => {
   await openhab.createThings(...thingsToImport);
 
   return thingsToImport;
-}
\ No newline at end of file
+}