diff --git a/src/main/java/com/google/devtools/build/lib/syntax/BinaryOperatorExpression.java b/src/main/java/com/google/devtools/build/lib/syntax/BinaryOperatorExpression.java
index 7b8c2df..5624ba7 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/BinaryOperatorExpression.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/BinaryOperatorExpression.java
@@ -391,6 +391,7 @@
       case IN: {
         if (rtype.isList()
             || rtype.isSet()
+            || rtype.isNset()
             || rtype.isDict()
             || rtype == SkylarkType.STRING) {
           return SkylarkType.BOOL;
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java b/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java
index db47743..9f4cd83 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java
@@ -632,7 +632,7 @@
       return toCollection(o, loc);
     } else {
       throw new EvalException(loc,
-          "type '" + getDataTypeName(o) + "' is not an iterable");
+          "type '" + getDataTypeName(o) + "' is not iterable");
     }
   }
 
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/LValue.java b/src/main/java/com/google/devtools/build/lib/syntax/LValue.java
index d2e3925..e7f15d7 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/LValue.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/LValue.java
@@ -126,7 +126,7 @@
       return;
     }
     throw new EvalException(loc,
-        "can only assign to variables, not to '" + expr + "'");
+        "can only assign to variables and tuples, not to '" + expr + "'");
   }
 
   @Override
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkEnvironment.java b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkEnvironment.java
index bfb7336..b264dd6 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkEnvironment.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkEnvironment.java
@@ -187,16 +187,6 @@
   }
 
   /**
-   * Updates the value of variable "varname" in the environment, corresponding
-   * to an AssignmentStatement.
-   */
-  @Override
-  public void update(String varname, Object value) {
-    Preconditions.checkNotNull(value, "update(value == null)");
-    env.put(varname, value);
-  }
-
-  /**
    * Returns the class of the variable or null if the variable does not exist. This function
    * works only in the local Environment, it doesn't check the global Environment.
    */
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkType.java b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkType.java
index ab740a4..92ae48b 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkType.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkType.java
@@ -107,7 +107,7 @@
 
   /** @return true if any object of this SkylarkType can be cast to that Java class */
   public boolean canBeCastTo(Class<?> type) {
-    return Simple.of(type).includes(this);
+    return SkylarkType.of(type).includes(this);
   }
 
   /** @return true if some non-null objects of that Java class can be cast to this SkylarkType
@@ -498,6 +498,8 @@
       return LIST;
     } else if (SkylarkNestedSet.class.isAssignableFrom(type)) {
       return SET;
+    } else if (Function.class.isAssignableFrom(type)) {
+      return new SkylarkFunctionType("unknown", TOP);
     } else {
       return Simple.of(type);
     }
@@ -776,11 +778,7 @@
   /** Build a map of the given key, value types from an Iterable of Map.Entry-s */
   public static <KEY_TYPE, VALUE_TYPE> ImmutableMap<KEY_TYPE, VALUE_TYPE> toMap(
       Iterable<Map.Entry<KEY_TYPE, VALUE_TYPE>> obj) {
-    ImmutableMap.Builder<KEY_TYPE, VALUE_TYPE> builder = ImmutableMap.builder();
-    for (Map.Entry<KEY_TYPE, VALUE_TYPE> entry : obj) {
-      builder.put(entry.getKey(), entry.getValue());
-    }
-    return builder.build();
+    return ImmutableMap.copyOf(obj);
   }
 
   /**
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/ValidationEnvironment.java b/src/main/java/com/google/devtools/build/lib/syntax/ValidationEnvironment.java
index a3c1991..95d7f30 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/ValidationEnvironment.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/ValidationEnvironment.java
@@ -106,7 +106,7 @@
   /**
    * Updates the variable type if the new type is "stronger" then the old one.
    * The old and the new vartype has to be compatible, otherwise an EvalException is thrown.
-   * The new type is stronger if the old one doesn't exist or unknown.
+   * The new type is stronger if the old one doesn't exist or is unknown.
    */
   public void update(String varname, SkylarkType newVartype, Location location)
       throws EvalException {
@@ -192,8 +192,9 @@
       SkylarkType functionType = functions.get(funcName);
       if (functionType != null && functionType != SkylarkType.UNKNOWN) {
         if (!(functionType instanceof SkylarkFunctionType)) {
-          throw new EvalException(loc, (objectType == SkylarkType.GLOBAL ? "" : objectType + ".")
-              + funcName + " is not a function");
+          throw new EvalException(loc, String.format("%s%s is not a function but a(n) %s",
+                  (objectType == SkylarkType.GLOBAL ? "" : objectType + "."),
+                  funcName, functionType));
         }
         return ((SkylarkFunctionType) functionType).getReturnType();
       }