diff --git a/Models/ThemeConfig/Conversion/Conversion.ts b/Models/ThemeConfig/Conversion/Conversion.ts
index 3d9aa1183a..5b6b72c008 100644
--- a/Models/ThemeConfig/Conversion/Conversion.ts
+++ b/Models/ThemeConfig/Conversion/Conversion.ts
@@ -216,13 +216,18 @@ export class Fuse<T> extends DesugaringStep<T> {
         const information = []
         for (let i = 0; i < this.steps.length; i++) {
             const step = this.steps[i];
-            let r = step.convert(json, "While running step " + step.name + ": " + context)
-            errors.push(...r.errors ?? [])
-            warnings.push(...r.warnings ?? [])
-            information.push(...r.information ?? [])
-            json = r.result
-            if (errors.length > 0) {
-                break;
+            try{
+                let r = step.convert(json, "While running step " + step.name + ": " + context)
+                errors.push(...r.errors ?? [])
+                warnings.push(...r.warnings ?? [])
+                information.push(...r.information ?? [])
+                json = r.result
+                if (errors.length > 0) {
+                    break;
+                }
+            }catch(e){
+                console.error("Step "+step.name+" failed due to "+e);
+                throw e
             }
         }
         return {
diff --git a/Models/ThemeConfig/Conversion/PrepareTheme.ts b/Models/ThemeConfig/Conversion/PrepareTheme.ts
index 7c1aa17211..8a9877e108 100644
--- a/Models/ThemeConfig/Conversion/PrepareTheme.ts
+++ b/Models/ThemeConfig/Conversion/PrepareTheme.ts
@@ -479,6 +479,7 @@ export class PrepareTheme extends Fuse<LayoutConfigJson> {
     }) {
         super(
             "Fully prepares and expands a theme",
+
             new AddContextToTransltionsInLayout(),
             new PreparePersonalTheme(state),
             new WarnForUnsubstitutedLayersInTheme(),
diff --git a/Models/ThemeConfig/Conversion/Validation.ts b/Models/ThemeConfig/Conversion/Validation.ts
index 912707877c..b0c121953b 100644
--- a/Models/ThemeConfig/Conversion/Validation.ts
+++ b/Models/ThemeConfig/Conversion/Validation.ts
@@ -217,12 +217,17 @@ class MiscThemeChecks extends DesugaringStep<LayoutConfigJson>{
     
     convert(json: LayoutConfigJson, context: string): { result: LayoutConfigJson; errors?: string[]; warnings?: string[]; information?: string[] } {
         const warnings = []
+        const errors = []
+        if(json.id !== "personal" && (json.layers === undefined || json.layers.length === 0)){
+            errors.push("The theme "+json.id+" has no 'layers' defined ("+context+")")
+        }
         if(json.socialImage === ""){
             warnings.push("Social image for theme "+json.id+" is the emtpy string")
         }
         return {
             result :json,
-            warnings
+            warnings,
+            errors
         };
     }
 }
@@ -231,8 +236,8 @@ export class PrevalidateTheme extends Fuse<LayoutConfigJson> {
 
     constructor() {
         super("Various consistency checks on the raw JSON",
-            new OverrideShadowingCheck(),
-            new MiscThemeChecks()
+            new MiscThemeChecks(),
+            new OverrideShadowingCheck()
         );
 
     }
diff --git a/Models/ThemeConfig/Json/UnitConfigJson.ts b/Models/ThemeConfig/Json/UnitConfigJson.ts
index 5eba5eaf49..bde2683b23 100644
--- a/Models/ThemeConfig/Json/UnitConfigJson.ts
+++ b/Models/ThemeConfig/Json/UnitConfigJson.ts
@@ -18,9 +18,12 @@ export default interface UnitConfigJson {
 
 export interface ApplicableUnitJson {
     /**
-     * The canonical value which will be added to the text.
+     * The canonical value which will be added to the value in OSM.
      * e.g. "m" for meters
-     * If the user inputs '42', the canonical value will be added and it'll become '42m'
+     * If the user inputs '42', the canonical value will be added and it'll become '42m'.
+     * 
+     * Important: often, _no_ canonical values are expected, e.g. in the case of 'maxspeed' where 'km/h' is the default.
+     * In this case, an empty string should be used
      */
     canonicalDenomination: string,
     /**
diff --git a/Models/ThemeConfig/LayerConfig.ts b/Models/ThemeConfig/LayerConfig.ts
index b7863b4274..ade6d6b6df 100644
--- a/Models/ThemeConfig/LayerConfig.ts
+++ b/Models/ThemeConfig/LayerConfig.ts
@@ -134,6 +134,9 @@ export default class LayerConfig extends WithContextLoader {
 
         this.allowSplit = json.allowSplit ?? false;
         this.name = Translations.T(json.name, translationContext + ".name");
+        if(json.units!==undefined && !Array.isArray(json.units)){
+            throw "At "+context+".units: the 'units'-section should be a list; you probably have an object there"
+        }
         this.units = (json.units ?? []).map(((unitJson, i) => Unit.fromJson(unitJson, `${context}.unit[${i}]`)))
 
         if (json.description !== undefined) {
diff --git a/scripts/generateLayerOverview.ts b/scripts/generateLayerOverview.ts
index d9faf2d9ab..02cc138594 100644
--- a/scripts/generateLayerOverview.ts
+++ b/scripts/generateLayerOverview.ts
@@ -203,16 +203,22 @@ class LayerOverviewUtils {
             const themePath = themeInfo.path
 
             new PrevalidateTheme().convertStrict(themeFile, themePath)
-            themeFile = new PrepareTheme(convertState).convertStrict(themeFile, themePath)
-
-            if(knownImagePaths === undefined){
-                throw "Could not load known images/licenses"
+            try{
+                
+                themeFile = new PrepareTheme(convertState).convertStrict(themeFile, themePath)
+    
+                if(knownImagePaths === undefined){
+                    throw "Could not load known images/licenses"
+                }
+                new ValidateThemeAndLayers(knownImagePaths, themePath, true, convertState.tagRenderings)
+                    .convertStrict(themeFile, themePath)
+    
+                this.writeTheme(themeFile)
+                fixed.set(themeFile.id, themeFile)
+            }catch(e){
+                console.error("ERROR: could not prepare theme "+themePath+" due to "+e)
+                throw e;
             }
-            new ValidateThemeAndLayers(knownImagePaths, themePath, true, convertState.tagRenderings)
-                .convertStrict(themeFile, themePath)
-
-            this.writeTheme(themeFile)
-            fixed.set(themeFile.id, themeFile)
         }
 
         this.writeSmallOverview(themeFiles.map(tf => {