diff --git a/src/SHAMap.h b/src/SHAMap.h
index 2e1af1dcaf..64eec583a8 100644
--- a/src/SHAMap.h
+++ b/src/SHAMap.h
@@ -157,11 +157,12 @@ public:
 	SHAMapTreeNode(const SHAMapTreeNode& node, uint32 seq); // copy node from older tree
 	SHAMapTreeNode(const SHAMapNode& nodeID, SHAMapItem::pointer item, TNType type, uint32 seq);
 
-	// raw node functions
-	SHAMapTreeNode(const SHAMapNode& id, const std::vector<unsigned char>& contents, uint32 seq, int format);
-
+#define STN_ARF_UNKNOWN		0
 #define STN_ARF_PREFIXED	1
 #define STN_ARF_WIRE		2
+
+	// raw node functions
+	SHAMapTreeNode(const SHAMapNode& id, const std::vector<unsigned char>& contents, uint32 seq, int format);
 	void addRaw(Serializer &, int format);
 
 	virtual bool isPopulated() const { return true; }
diff --git a/src/SHAMapNodes.cpp b/src/SHAMapNodes.cpp
index dbdc8ee109..3a9c682414 100644
--- a/src/SHAMapNodes.cpp
+++ b/src/SHAMapNodes.cpp
@@ -192,6 +192,14 @@ SHAMapTreeNode::SHAMapTreeNode(const SHAMapNode& node, SHAMapItem::pointer item,
 SHAMapTreeNode::SHAMapTreeNode(const SHAMapNode& id, const std::vector<unsigned char>& rawNode, uint32 seq, int format)
 	: SHAMapNode(id), mSeq(seq), mType(tnERROR), mFullBelow(false)
 {
+	if (format == STN_ARF_UNKNOWN)
+	{
+		if ((rawNode.size() < 4) || (rawNode[0] < 10))
+			format = STN_ARF_WIRE;
+		else
+			format = STN_ARF_PREFIXED;
+	}
+
 	if (format == STN_ARF_WIRE)
 	{
 		Serializer s(rawNode);