diff --git a/core/commonMain/src/kotlinx/serialization/internal/Enums.kt b/core/commonMain/src/kotlinx/serialization/internal/Enums.kt
index 5644bd4c5..1e6201546 100644
--- a/core/commonMain/src/kotlinx/serialization/internal/Enums.kt
+++ b/core/commonMain/src/kotlinx/serialization/internal/Enums.kt
@@ -67,16 +67,22 @@ internal class EnumSerializer<T : Enum<T>>(
 
     override fun serialize(encoder: Encoder, value: T) {
         val index = values.indexOf(value)
-        check(index != -1) {
-            "$value is not a valid enum ${descriptor.serialName}, must be one of ${values.contentToString()}"
+        if (index == -1) {
+            throw SerializationException(
+                "$value is not a valid enum ${descriptor.serialName}, " +
+                        "must be one of ${values.contentToString()}"
+            )
         }
         encoder.encodeEnum(descriptor, index)
     }
 
     override fun deserialize(decoder: Decoder): T {
         val index = decoder.decodeEnum(descriptor)
-        check(index in values.indices) {
-            "$index is not among valid $${descriptor.serialName} enum values, values size is ${values.size}"
+        if (index !in values.indices) {
+            throw SerializationException(
+                "$index is not among valid ${descriptor.serialName} enum values, " +
+                        "values size is ${values.size}"
+            )
         }
         return values[index]
     }
diff --git a/formats/protobuf/commonMain/src/kotlinx/serialization/protobuf/internal/Streams.kt b/formats/protobuf/commonMain/src/kotlinx/serialization/protobuf/internal/Streams.kt
index 540b7b46d..da712fa40 100644
--- a/formats/protobuf/commonMain/src/kotlinx/serialization/protobuf/internal/Streams.kt
+++ b/formats/protobuf/commonMain/src/kotlinx/serialization/protobuf/internal/Streams.kt
@@ -97,7 +97,7 @@ internal class ByteArrayInput(private var array: ByteArray, private val endIndex
     private fun readVarint64SlowPath(): Long {
         var result = 0L
         var shift = 0
-        while (shift != 64) {
+        while (shift < 64) {
             val byte = read()
             result = result or ((byte and 0x7F).toLong() shl shift)
             if (byte and 0x80 == 0) {
@@ -105,13 +105,13 @@ internal class ByteArrayInput(private var array: ByteArray, private val endIndex
             }
             shift += 7
         }
-        throw SerializationException("Varint too long: exceeded 64 bits")
+        throw SerializationException("Input stream is malformed: Varint too long (exceeded 64 bits)")
     }
 
     private fun readVarint32SlowPath(): Int {
         var result = 0
         var shift = 0
-        while (shift != 32) {
+        while (shift < 32) {
             val byte = read()
             result = result or ((byte and 0x7F) shl shift)
             if (byte and 0x80 == 0) {
@@ -119,7 +119,7 @@ internal class ByteArrayInput(private var array: ByteArray, private val endIndex
             }
             shift += 7
         }
-        throw SerializationException("Varint too long: exceeded 32 bits")
+        throw SerializationException("Input stream is malformed: Varint too long (exceeded 32 bits)")
     }
 }
 
diff --git a/formats/protobuf/commonTest/src/kotlinx/serialization/protobuf/ProtobufEnumTest.kt b/formats/protobuf/commonTest/src/kotlinx/serialization/protobuf/ProtobufEnumTest.kt
new file mode 100644
index 000000000..188fb93a6
--- /dev/null
+++ b/formats/protobuf/commonTest/src/kotlinx/serialization/protobuf/ProtobufEnumTest.kt
@@ -0,0 +1,39 @@
+/*
+ * Copyright 2017-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.protobuf
+
+import kotlinx.serialization.*
+import kotlin.test.*
+
+class ProtobufEnumTest {
+
+    enum class SomeEnum { ALPHA, BETA, GAMMA }
+
+    @Serializable
+    data class EnumWithUnion(@ProtoNumber(5) val s: String,
+                             @ProtoNumber(6) val e: SomeEnum = SomeEnum.ALPHA,
+                             @ProtoNumber(7) val i: Int = 42)
+
+    @Test
+    fun testEnumWithUnion() {
+        val data = EnumWithUnion("foo", SomeEnum.BETA)
+        val hex = ProtoBuf.encodeToHexString(EnumWithUnion.serializer(), data)
+        val restored = ProtoBuf.decodeFromHexString(EnumWithUnion.serializer(), hex)
+        assertEquals(data, restored)
+    }
+
+    @Serializable
+    class EnumHolder(val e: SomeEnum)
+
+    @Test
+    fun testUnknownValue() {
+        val bytes = ProtoBuf.encodeToByteArray(EnumHolder(SomeEnum.ALPHA))
+        bytes[1] = 3
+        assertFailsWith<SerializationException> { ProtoBuf.decodeFromByteArray<EnumHolder>(bytes) }
+        bytes[1] = -1
+        assertFailsWith<SerializationException> { ProtoBuf.decodeFromByteArray<EnumHolder>(bytes) }
+
+    }
+}
diff --git a/formats/protobuf/commonTest/src/kotlinx/serialization/protobuf/ProtobufUnionEnumTest.kt b/formats/protobuf/commonTest/src/kotlinx/serialization/protobuf/ProtobufUnionEnumTest.kt
deleted file mode 100644
index 23cc2c0d4..000000000
--- a/formats/protobuf/commonTest/src/kotlinx/serialization/protobuf/ProtobufUnionEnumTest.kt
+++ /dev/null
@@ -1,26 +0,0 @@
-/*
- * Copyright 2017-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
- */
-
-package kotlinx.serialization.protobuf
-
-import kotlinx.serialization.*
-import kotlin.test.*
-
-class ProtobufUnionEnumTest {
-
-    enum class SomeEnum { ALPHA, BETA, GAMMA }
-
-    @Serializable
-    data class WithUnions(@ProtoNumber(5) val s: String,
-                          @ProtoNumber(6) val e: SomeEnum = SomeEnum.ALPHA,
-                          @ProtoNumber(7) val i: Int = 42)
-
-    @Test
-    fun testEnum() {
-        val data = WithUnions("foo", SomeEnum.BETA)
-        val hex = ProtoBuf.encodeToHexString(WithUnions.serializer(), data)
-        val restored = ProtoBuf.decodeFromHexString(WithUnions.serializer(), hex)
-        assertEquals(data, restored)
-    }
-}