diff --git a/src/common/ecs/compiler.ts b/src/common/ecs/compiler.ts
index 2f109608..13de3ed8 100644
--- a/src/common/ecs/compiler.ts
+++ b/src/common/ecs/compiler.ts
@@ -92,9 +92,11 @@ export class ECSCompiler extends Tokenizer {
     }
 
     annotate<T extends SourceLocated>(fn: () => T) {
-        let tok = this.peekToken();
+        let start = this.peekToken();
         let obj = fn();
-        if (obj) (obj as SourceLocated).$loc = tok.$loc;
+        let end = this.lasttoken;
+        let $loc = end ? mergeLocs(start.$loc, end.$loc) : start.$loc;
+        if (obj) (obj as SourceLocated).$loc = $loc;
         return obj;
     }
 
@@ -349,7 +351,7 @@ export class ECSCompiler extends Tokenizer {
             tempbytes = this.parseIntegerConstant();
         }
         let system: System = { name, tempbytes, actions: [] };
-        let expr = this.parseBlockStatement();
+        let expr = this.annotate(() => this.parseBlockStatement());
         let action: Action = { expr, event: name };
         system.actions.push(action);
         return system;
@@ -363,7 +365,7 @@ export class ECSCompiler extends Tokenizer {
         let critical = undefined;
         if (this.ifToken('critical')) critical = true;
         if (this.ifToken('fit')) fitbytes = this.parseIntegerConstant();
-        let expr = this.parseBlockStatement();
+        let expr = this.annotate(() => this.parseBlockStatement());
         //query, join, select, direction, 
         let action : Action = { expr, event, fitbytes, critical };
         return action as Action;
@@ -770,6 +772,7 @@ export class ECSCompiler extends Tokenizer {
     parseExprList(): Expr[] {
         return this.parseList(this.parseExpr, ',');
     }
+    // TODO: annotate with location
     parseBlockStatement(): Statement {
         let valtype : IntType = { dtype:'int', lo:0, hi: 0 } // TODO?
         if (this.peekToken().type == ECSTokenType.CodeFragment) {
@@ -778,7 +781,7 @@ export class ECSCompiler extends Tokenizer {
         if (this.ifToken('begin')) {
             let stmts = [];
             while (this.peekToken().str != 'end') {
-                stmts.push(this.parseBlockStatement());
+                stmts.push(this.annotate(() => this.parseBlockStatement()));
             }
             this.expectToken('end');
             return { valtype, stmts };
@@ -812,7 +815,7 @@ export class ECSCompiler extends Tokenizer {
         let direction = undefined;
         if (modifiers['asc']) direction = 'asc';
         else if (modifiers['desc']) direction = 'desc';
-        let body = this.parseBlockStatement();
+        let body = this.annotate(() => this.parseBlockStatement());
         return { select, query, join, direction, stmts: [body], loop: select == 'foreach' } as QueryExpr;
     }
 }
diff --git a/src/common/ecs/ecs.ts b/src/common/ecs/ecs.ts
index e60e91bf..c38b46e8 100644
--- a/src/common/ecs/ecs.ts
+++ b/src/common/ecs/ecs.ts
@@ -3,8 +3,10 @@ import { Token } from "../tokenizer";
 import { SourceLocated, SourceLocation } from "../workertypes";
 import { Bin, Packer } from "./binpack";
 
-export class ECSError extends Error {
+export class ECSError extends Error implements SourceLocated {
     $loc: SourceLocation;
+    $sources: SourceLocated[] = [];
+
     constructor(msg: string, obj?: SourceLocation | SourceLocated) {
         super(msg);
         Object.setPrototypeOf(this, ECSError.prototype);
@@ -834,7 +836,12 @@ class ActionEval {
     __emit(args: string[]) {
         let event = args[0];
         let eventargs = args.slice(1);
-        return this.scope.generateCodeForEvent(event, eventargs);
+        try {
+            return this.scope.generateCodeForEvent(event, eventargs);
+        } catch (e) {
+            if (e.$sources) e.$sources.push(this.action);
+            throw e;
+        }
     }
     __local(args: string[]) {
         let tempinc = parseInt(args[0]);
@@ -1053,6 +1060,7 @@ class ActionEval {
             case 'once':
                 // TODO: how is this different from begin/end?
                 //state.xreg = state.yreg = null;
+                //state.working = new EntitySet(scope, undefined, [], []);
                 break;
             case 'foreach':
             case 'unroll':
@@ -1077,7 +1085,7 @@ class ActionEval {
                         if (select == 'if') {
                             qr.entities = []; // "if" failed
                         } else {
-                            throw new ECSError(`no entities in statement`, action);
+                            throw new ECSError(`no entities match query`, qexpr);
                         }
                     } else {
                         // TODO: must be a better way...
diff --git a/src/worker/tools/ecs.ts b/src/worker/tools/ecs.ts
index 040d1b94..6798ca5a 100644
--- a/src/worker/tools/ecs.ts
+++ b/src/worker/tools/ecs.ts
@@ -27,6 +27,11 @@ export function assembleECS(step: BuildStep): BuildStepResult {
         } catch (e) {
             if (e instanceof ECSError) {
                 compiler.addError(e.message, e.$loc);
+                for (let obj of e.$sources) {
+                    let name = (obj as any).event;
+                    if (name == 'start') break;
+                    compiler.addError(`... ${name}`, obj.$loc); // TODO?
+                }
                 return { errors: compiler.errors };
             } else if (e instanceof CompileError) {
                 return { errors: compiler.errors };