diff --git a/src/org/rascalmpl/library/lang/json/internal/JsonValueReader.java b/src/org/rascalmpl/library/lang/json/internal/JsonValueReader.java
index ffc9c00d512..e43e658afa9 100644
--- a/src/org/rascalmpl/library/lang/json/internal/JsonValueReader.java
+++ b/src/org/rascalmpl/library/lang/json/internal/JsonValueReader.java
@@ -25,8 +25,6 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
-import java.util.stream.Collectors;
-
 import org.rascalmpl.debug.IRascalMonitor;
 import org.rascalmpl.uri.URIUtil;
 import io.usethesource.vallang.IInteger;
@@ -494,7 +492,6 @@ private int getCol() {
         }
       }
 
-
       @Override
       public IValue visitAbstractData(Type type) throws IOException {
         if (in.peek() == JsonToken.STRING) {
@@ -574,6 +571,9 @@ else if (explicitConstructorNames && consName == null) {
         if (alternatives.size() > 1) {
           monitor.warning("selecting arbitrary constructor for " + type, vf.sourceLocation(in.getPath()));
         }
+        else if (alternatives.size() == 0) {
+          throw new IOException("No fitting constructor found for " + in.getPath());
+        }
         Type cons = alternatives.iterator().next();
        
         
diff --git a/src/org/rascalmpl/library/lang/rascal/tests/library/lang/json/JSONIOTests.rsc b/src/org/rascalmpl/library/lang/rascal/tests/library/lang/json/JSONIOTests.rsc
index f94c3d06cff..0fd17e4119f 100644
--- a/src/org/rascalmpl/library/lang/rascal/tests/library/lang/json/JSONIOTests.rsc
+++ b/src/org/rascalmpl/library/lang/rascal/tests/library/lang/json/JSONIOTests.rsc
@@ -91,10 +91,15 @@ test bool explicitDataTypes() {
     
     assert json == "{\"_constructor\":\"data4\",\"_type\":\"DATA4\",\"e\":{\"_constructor\":\"z\",\"_type\":\"Enum\"}}";
 
+    // _constructor and _type must be the first fields
     assert parseJSON(#DATA4, json, explicitDataTypes=true) == example;
 
+    // _type and _constructor may appear in a different order
+    flippedJson = "{\"_type\":\"DATA4\",\"_constructor\":\"data4\",\"e\":{\"_constructor\":\"z\",\"_type\":\"Enum\"}}";
+    assert parseJSON(#DATA4, flippedJson, explicitDataTypes=true) == example;
+
     // here we can't be sure to get z() back, but we will get some Enum
-    // assert data4(e=Enum _) := parseJSON(#DATA4, json, explicitDataTypes=false);
+    assert data4(e=Enum _) := parseJSON(#DATA4, json, explicitDataTypes=false);
 
     return true;
 }
\ No newline at end of file