diff --git a/Z85.js b/Z85.js
index c9c8b68..a86a67b 100644
--- a/Z85.js
+++ b/Z85.js
@@ -41,18 +41,18 @@
         '}', '@', '%', '$', '#'
     ];
     const _z85DecodeTable = [
-        0x00, 0x44, 0x00, 0x54, 0x53, 0x52, 0x48, 0x00,
-        0x4B, 0x4C, 0x46, 0x41, 0x00, 0x3F, 0x3E, 0x45,
+          -1, 0x44,   -1, 0x54, 0x53, 0x52, 0x48,   -1,
+        0x4B, 0x4C, 0x46, 0x41,   -1, 0x3F, 0x3E, 0x45,
         0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
-        0x08, 0x09, 0x40, 0x00, 0x49, 0x42, 0x4A, 0x47,
+        0x08, 0x09, 0x40,   -1, 0x49, 0x42, 0x4A, 0x47,
         0x51, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29, 0x2A,
         0x2B, 0x2C, 0x2D, 0x2E, 0x2F, 0x30, 0x31, 0x32,
         0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x3A,
-        0x3B, 0x3C, 0x3D, 0x4D, 0x00, 0x4E, 0x43, 0x00,
-        0x00, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F, 0x10,
+        0x3B, 0x3C, 0x3D, 0x4D,   -1, 0x4E, 0x43,   -1,
+          -1, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F, 0x10,
         0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18,
         0x19, 0x1A, 0x1B, 0x1C, 0x1D, 0x1E, 0x1F, 0x20,
-        0x21, 0x22, 0x23, 0x4F, 0x00, 0x50, 0x00, 0x00
+        0x21, 0x22, 0x23, 0x4F,   -1, 0x50,   -1,   -1
     ];
     return {
         /**
@@ -152,8 +152,10 @@
             let charIdx = 0;
             let byteIdx = 0;
             for (let i = 0; i < length; ++i) {
-                const code = string.charCodeAt(charIdx++) - 32;
-                value = value * 85 + _z85DecodeTable[code];
+                const code = _z85DecodeTable[string.charCodeAt(charIdx++) - 32];
+                if (!(code >= 0)) // also manages undefined values
+                    throw new Error("Character '" + string.charAt(i) + "' in position " + charIdx + " is not valid Z85");
+                value = value * 85 + code;
                 if (charIdx % 5 === 0) {
                     let divisor = 256 * 256 * 256;
                     while (divisor >= 1) {
diff --git a/test/test.js b/test/test.js
index 93a66ba..1dfcc3e 100644
--- a/test/test.js
+++ b/test/test.js
@@ -57,4 +57,19 @@ describe('Z85', () => {
 
         done();
     });
+    [
+        { name: 'invalid ASCII',    encoded: 'a_b', expected: "Character '_' in position 2 is not valid Z85" },
+        { name: 'invalid Uniucode', encoded: '12Ø', expected: "Character 'Ø' in position 3 is not valid Z85" },
+    ].forEach(test => {
+        it('properly chokes on ' + test.name, done => {
+            let err = null;
+            try {
+                Z85.decode(test.encoded);
+            } catch (e) {
+                err = e;
+            }
+            assert.strictEqual(err.message, test.expected);
+            done();
+        });
+    });
 });