diff --git a/src/shamira.py b/src/shamira.py
--- a/src/shamira.py
+++ b/src/shamira.py
@@ -8,9 +8,16 @@ import binascii
 import gf256
 
 
+class SException(Exception): pass
+class InvalidParams(SException): pass
+class DetectionException(SException): pass
+class DecodingException(SException): pass
+class MalformedShare(SException): pass
+
+
 def _shareByte(secretB,k,n):
 	if not k<=n<255:
-		raise ValueError("failing k<=n<255, k={0}, n={1}".format(k,n))
+		raise InvalidParams("Failed k<=n<255, k={0}, n={1}".format(k,n))
 	coefs=[int(secretB)]+[int(b) for b in os.urandom(k-1)]
 	points=[gf256.evaluate(coefs,i) for i in range(1,n+1)]
 	return points
@@ -65,7 +72,10 @@ def reconstruct(*shares,encoding="",raw=
 		encoding=detectEncoding(shares)
 
 	bs=reconstructRaw(*(decode(s,encoding) for s in shares))
-	return bs if raw else bs.decode(encoding="utf-8")
+	try:
+		return bs if raw else bs.decode(encoding="utf-8")
+	except UnicodeDecodeError:
+		raise DecodingException('Failed to decode bytes to utf-8. Either you supplied invalid shares, or you missed the "raw" flag. Offending value: {0}'.format(bs))
 
 
 def encode(share,encoding="b32"):
@@ -81,14 +91,14 @@ def decode(share,encoding="b32"):
 		(i,_,shareStr)=share.partition(".")
 		i=int(i)
 		if not 1<=i<=255:
-			raise ValueError()
+			raise MalformedShare("Malformed share: Failed 1<=k<=255, k={0}".format(i))
 		if encoding=="hex": f=base64.b16decode
 		elif encoding=="b32": f=base64.b32decode
 		else: f=base64.b64decode
 		shareBytes=f(shareStr)
 		return (i,shareBytes)
 	except (ValueError,binascii.Error):
-		raise ValueError('bad share format: share="{0}", encoding="{1}"'.format(share,encoding))
+		raise MalformedShare('Malformed share: share="{0}", encoding="{1}"'.format(share,encoding))
 
 
 def detectEncoding(shares):
@@ -100,7 +110,7 @@ def detectEncoding(shares):
 	for (regexp, res) in classes:
 		if all(regexp.fullmatch(share) for share in shares):
 			return res
-	raise ValueError("no expected encoding detected")
+	raise DetectionException("No expected encoding detected")
 
 
 if __name__=="__main__":