mongodb
diff --git a/‎generator/config/expressions.php
+6 b/‎generator/config/expressions.php
+6
diff --git a/‎src/Builder/Accumulator/AccumulatorAccumulator.php
+8-8 b/‎src/Builder/Accumulator/AccumulatorAccumulator.php
+8-8
diff --git a/‎src/Builder/Accumulator/AvgAccumulator.php
+4-4 b/‎src/Builder/Accumulator/AvgAccumulator.php
+4-4
diff --git a/‎src/Builder/Accumulator/BottomNAccumulator.php
+4-4 b/‎src/Builder/Accumulator/BottomNAccumulator.php
+4-4
diff --git a/‎src/Builder/Accumulator/CovariancePopAccumulator.php
+8-8 b/‎src/Builder/Accumulator/CovariancePopAccumulator.php
+8-8
diff --git a/‎src/Builder/Accumulator/CovarianceSampAccumulator.php
+8-8 b/‎src/Builder/Accumulator/CovarianceSampAccumulator.php
+8-8
diff --git a/‎src/Builder/Accumulator/DerivativeAccumulator.php
+4-4 b/‎src/Builder/Accumulator/DerivativeAccumulator.php
+4-4
diff --git a/‎src/Builder/Accumulator/ExpMovingAvgAccumulator.php
+12-12 b/‎src/Builder/Accumulator/ExpMovingAvgAccumulator.php
+12-12
@@ -13,6 +13,7 @@
 use MongoDB\Model\BSONArray;
 use stdClass;
 
+use function in_array;
 use function ucfirst;
 
 $bsonTypes = [
@@ -45,6 +46,11 @@
 $expressions = [];
 $resolvesToInterfaces = [];
 foreach ($bsonTypes as $name => $acceptedTypes) {
+    // an expression can be a string with a $-prefixed field name
+    if (! in_array('string', $acceptedTypes)) {
+        $acceptedTypes[] = 'string';
+    }
+
     $expressions[$name] = ['acceptedTypes' => $acceptedTypes];
 
     $resolvesTo = 'resolvesTo' . ucfirst($name);