diff --git a/src/commonMain/kotlin/com/infendro/encoding/exception/DecodeException.kt b/src/commonMain/kotlin/com/infendro/encoding/exception/DecodeException.kt
new file mode 100644
index 0000000..f81ba30
--- /dev/null
+++ b/src/commonMain/kotlin/com/infendro/encoding/exception/DecodeException.kt
@@ -0,0 +1,5 @@
+package com.infendro.encoding.exception
+
+abstract class DecodeException : Exception() {
+    abstract override val message: String
+}
diff --git a/src/commonMain/kotlin/com/infendro/encoding/exception/IllegalByteException.kt b/src/commonMain/kotlin/com/infendro/encoding/exception/IllegalByteException.kt
new file mode 100644
index 0000000..5650bf6
--- /dev/null
+++ b/src/commonMain/kotlin/com/infendro/encoding/exception/IllegalByteException.kt
@@ -0,0 +1,12 @@
+package com.infendro.encoding.exception
+
+class IllegalByteException(
+    val byte: Byte,
+) : DecodeException() {
+    override val message: String
+        get() {
+            val hex = "0x${byte.toHexString().padStart(2, '0')}"
+            val char = "'${byte.toInt().toChar()}'"
+            return "Illegal byte $hex ($char)."
+        }
+}
diff --git a/src/commonMain/kotlin/com/infendro/encoding/numeric/NumericEncoding.kt b/src/commonMain/kotlin/com/infendro/encoding/numeric/NumericEncoding.kt
index 16b36d4..671a552 100644
--- a/src/commonMain/kotlin/com/infendro/encoding/numeric/NumericEncoding.kt
+++ b/src/commonMain/kotlin/com/infendro/encoding/numeric/NumericEncoding.kt
@@ -1,6 +1,7 @@
 package com.infendro.encoding.numeric
 
 import com.infendro.encoding.Encoding
+import com.infendro.encoding.exception.IllegalByteException
 import com.infendro.encoding.util.BigInt
 import com.infendro.encoding.util.log2
 import kotlin.math.ceil
@@ -30,8 +31,11 @@ open class NumericEncoding(
 
         var value = BigInt.zero
         for (i in 0..<bytes.size) {
-            val index = alphabet.indexOf(bytes[i])
-            if (index == -1) throw Exception() //TODO
+            val byte = bytes[i]
+
+            val index = alphabet.indexOf(byte)
+            if (index == -1) throw IllegalByteException(byte)
+
             value = value.timesAdd(base, index)
         }
 
diff --git a/src/commonMain/kotlin/com/infendro/encoding/stream/StreamEncoding.kt b/src/commonMain/kotlin/com/infendro/encoding/stream/StreamEncoding.kt
index 6c9e02a..6bce001 100644
--- a/src/commonMain/kotlin/com/infendro/encoding/stream/StreamEncoding.kt
+++ b/src/commonMain/kotlin/com/infendro/encoding/stream/StreamEncoding.kt
@@ -1,6 +1,7 @@
 package com.infendro.encoding.stream
 
 import com.infendro.encoding.Encoding
+import com.infendro.encoding.exception.IllegalByteException
 import com.infendro.encoding.util.align
 import com.infendro.encoding.util.divCeil
 import com.infendro.encoding.util.lcm
@@ -76,7 +77,7 @@ open class StreamEncoding(
             if (byte == padding) break
 
             val index = alphabet.indexOf(byte)
-            if (index == -1) throw Exception() //TODO
+            if (index == -1) throw IllegalByteException(byte)
 
             buffer = (buffer shl bits) + index.toByte()
             bufferBits += bits