diff --git a/src/Models/ThemeConfig/Conversion/Conversion.ts b/src/Models/ThemeConfig/Conversion/Conversion.ts
index dc6fd7d7e..23db13625 100644
--- a/src/Models/ThemeConfig/Conversion/Conversion.ts
+++ b/src/Models/ThemeConfig/Conversion/Conversion.ts
@@ -166,11 +166,11 @@ export class Each<X, Y> extends Conversion<X[], Y[]> {
     }
 }
 
-export class On<P, T> extends DesugaringStep<T> {
-    private readonly key: string
+export class On<S extends string, P, T extends Record<S, P>> extends DesugaringStep<T> {
+    private readonly key: S
     private readonly step: (t: T) => Conversion<P, P>
 
-    constructor(key: string, step: Conversion<P, P> | ((t: T) => Conversion<P, P>)) {
+    constructor(key: S, step: Conversion<P, P> | ((t: T) => Conversion<P, P>)) {
         super(`On(${key}, ${step.name})`, "Applies " + step.name + " onto property `" + key + "`")
         if (typeof step === "function") {
             this.step = step
@@ -181,7 +181,7 @@ export class On<P, T> extends DesugaringStep<T> {
     }
 
     convert(json: T, context: ConversionContext): T {
-        const key = this.key
+        const key: S = this.key
         const value: P = json?.[key]
         if (value === undefined || value === null) {
             return json
@@ -189,7 +189,8 @@ export class On<P, T> extends DesugaringStep<T> {
 
         json = { ...json }
         const step = this.step(json)
-        json[key] = step.convert(value, context.enter(key).inOperation("on[" + key + "]"))
+        const converted = step.convert(value, context.enter(key).inOperation("on[" + key + "]"))
+        json[key] = <T[S]> converted
         return json
     }
 }
diff --git a/src/Models/ThemeConfig/Conversion/PrevalidateLayer.ts b/src/Models/ThemeConfig/Conversion/PrevalidateLayer.ts
index 623e073b3..08b267278 100644
--- a/src/Models/ThemeConfig/Conversion/PrevalidateLayer.ts
+++ b/src/Models/ThemeConfig/Conversion/PrevalidateLayer.ts
@@ -329,15 +329,15 @@ export class PrevalidateLayer extends DesugaringStep<LayerConfigJson> {
             }
         }
 
-        if (json.filter) {
-            new On("filter", new Each(new ValidateFilter())).convert(json, context)
+        if (json.filter && Array.isArray(json.filter)) {
+            new On("filter", new Each(new ValidateFilter())).convert(<any> json, context)
         }
 
         if (json.tagRenderings !== undefined) {
             new On(
                 "tagRenderings",
                 new Each(new ValidateTagRenderings(json, this._doesImageExist))
-            ).convert(json, context)
+            ).convert(<any> json, context)
         }
 
         if (json.pointRendering !== null && json.pointRendering !== undefined) {
diff --git a/src/Models/ThemeConfig/Conversion/Validation.ts b/src/Models/ThemeConfig/Conversion/Validation.ts
index a3f7f4b1a..010efe3f7 100644
--- a/src/Models/ThemeConfig/Conversion/Validation.ts
+++ b/src/Models/ThemeConfig/Conversion/Validation.ts
@@ -901,12 +901,12 @@ export class ValidateLayer extends Conversion<
     }
 }
 
-export class ValidateFilter extends DesugaringStep<FilterConfigJson> {
+export class ValidateFilter extends DesugaringStep<string | FilterConfigJson> {
     constructor() {
         super("ValidateFilter", "Detect common errors in the filters")
     }
 
-    convert(filter: FilterConfigJson, context: ConversionContext): FilterConfigJson {
+    convert(filter: string | FilterConfigJson, context: ConversionContext): string | FilterConfigJson {
         if (typeof filter === "string") {
             // Calling another filter, we skip
             return filter