diff --git a/lib/source-map-generator.js b/lib/source-map-generator.js
index 8111e061..c756e7f7 100644
--- a/lib/source-map-generator.js
+++ b/lib/source-map-generator.js
@@ -263,37 +263,73 @@ class SourceMapGenerator {
    * in to one of these categories.
    */
   _validateMapping(aGenerated, aOriginal, aSource, aName) {
-    // When aOriginal is truthy but has empty values for .line and .column,
-    // it is most likely a programmer error. In this case we throw a very
-    // specific error message to try to guide them the right way.
-    // For example: https://github.com/Polymer/polymer-bundler/pull/519
-    if (aOriginal && typeof aOriginal.line !== "number" && typeof aOriginal.column !== "number") {
-        throw new Error(
-            "original.line and original.column are not numbers -- you probably meant to omit " +
-            "the original mapping entirely and only map the generated position. If so, pass " +
-            "null for the original mapping instead of an object with empty or null values."
-        );
-    }
-
-    if (aGenerated && "line" in aGenerated && "column" in aGenerated
-        && aGenerated.line > 0 && aGenerated.column >= 0
-        && !aOriginal && !aSource && !aName) {
+    if (
+      aGenerated &&
+      typeof aGenerated.line === "number" &&
+      typeof aGenerated.column === "number" &&
+      aGenerated.line > 0 &&
+      aGenerated.column >= 0 &&
+      !aOriginal &&
+      !aSource &&
+      !aName
+    ) {
       // Case 1.
-
-    } else if (aGenerated && "line" in aGenerated && "column" in aGenerated
-             && aOriginal && "line" in aOriginal && "column" in aOriginal
-             && aGenerated.line > 0 && aGenerated.column >= 0
-             && aOriginal.line > 0 && aOriginal.column >= 0
-             && aSource) {
+      return;
+    } else if (
+      aGenerated &&
+      typeof aGenerated.line === "number" &&
+      typeof aGenerated.column === "number" &&
+      aOriginal &&
+      typeof aOriginal.line === "number" &&
+      typeof aOriginal.column === "number" &&
+      aGenerated.line > 0 &&
+      aGenerated.column >= 0 &&
+      aOriginal.line > 0 &&
+      aOriginal.column >= 0 &&
+      aSource
+    ) {
       // Cases 2 and 3.
-
+      return;
     } else {
-      throw new Error("Invalid mapping: " + JSON.stringify({
-        generated: aGenerated,
-        source: aSource,
-        original: aOriginal,
-        name: aName
-      }));
+      let errMsg;
+
+      // When `aOriginal` is truthy but has empty values for .line and .column,
+      // it is most likely a programmer error. In this case we throw a very
+      // specific error message to try to guide them the right way.
+      // For example: https://github.com/Polymer/polymer-bundler/pull/519
+      if (
+        aOriginal &&
+        (typeof aOriginal.line !== "number" ||
+        typeof aOriginal.column !== "number")
+      ) {
+        errMsg = 
+            "original.line and/or original.column are not numbers -- you probably meant to omit " +
+            "the original mapping entirely and only map the generated position. If so, pass " +
+            "null for the original mapping instead of an object with empty or null values.";
+      }
+
+      // When `aGenerated` is truthy but has empty values for .line and .column,
+      // it is most likely a programmer error. In this case we throw a very
+      // specific error message to try to guide them the right way.
+      else if (
+        aGenerated &&
+        (typeof aGenerated.line !== "number" ||
+        typeof aGenerated.column !== "number")
+      ) {
+        errMsg = 
+          "generated.line and/or generated.column are not numbers -- you MUST specify " +
+            "a valid generated position.";
+      }
+
+      throw new Error(
+        (errMsg ? errMsg + " " : "") + "Invalid mapping: " +
+          JSON.stringify({
+            generated: aGenerated,
+            source: aSource,
+            original: aOriginal,
+            name: aName
+          })
+      );
     }
   }