diff --git a/src/sgfParser/collection.py b/src/sgfParser/collection.py
--- a/src/sgfParser/collection.py
+++ b/src/sgfParser/collection.py
@@ -1,17 +1,19 @@
 from sgfParser.node import Node
-from . import skipWhitespace
+from . import skipWhitespace, ParserError
 
 
 class Collection:
 	def __init__(self,s):
 		self.gameTrees=[]
-		i,x=GameTree.create(s,0)
-		if x is None:
-			print("error when parsing Collection")
-			return
-		while x is not None:
+		i=skipWhitespace(s,0)
+		if i>=len(s): return
+		elif not GameTree.fits(s,i):
+			raise ParserError("expected a GameTree starting with '('",s,i)
+		while GameTree.fits(s,i):
+			i,x=GameTree.create(s,i)
 			self.gameTrees.append(x)
-			i,x=GameTree.create(s,i)
+		if i<len(s):
+			raise ParserError("expected EOF",s,i)
 
 	def listGames(self):
 		for tree in self.gameTrees:
@@ -24,38 +26,38 @@ class GameTree:
 		self.branches=[]
 
 	@staticmethod
+	def fits(s,i):
+		return i<len(s) and s[i]=="("
+
+	@staticmethod
 	def create(s,start):
+		assert GameTree.fits(s,start)
 		res=GameTree()
-		i=skipWhitespace(s,start)
-		if i>=len(s) or s[i]!="(":
-			# print("error when parsing GameTree")
-			return (start,None)
-		i,x=Node.create(s,i+1)
-		if x is None:
-			# print("error when parsing GameTree")
-			return (i,None)
+
+		i=skipWhitespace(s,start+1)
+		if not Node.fits(s,i):
+			raise ParserError("expected a Node starting with ';'",s,i)
+
 		y=None
-		while x is not None:
+		while Node.fits(s,i):
+			i,x=Node.create(s,i)
 			res.nodes.append(x)
 			if y: y.addChild(x)
 			x.setParent(y)
 			y=x
 			i=skipWhitespace(s,i)
-			i,x=Node.create(s, i)
-		i=skipWhitespace(s,i)
-		i,x=GameTree.create(s,i)
-		while x is not None:
+
+		while GameTree.fits(s,i):
+			i,x=GameTree.create(s,i)
 			res.branches.append(x)
 			subroot=x.getNode(0)
-			if subroot:
-				subroot.setParent(y)
+			subroot.setParent(y)
 			if y: y.addChild(subroot)
 			i=skipWhitespace(s,i)
-			i,x=GameTree.create(s,i)
-		if s[i]!=")":
-			# print("error when parsing GameTree")
-			return (i,None)
-		return (i+1,res)
+		if i>=len(s) or s[i]!=")":
+			raise ParserError("expected end of the GameTree marked by ')'",s,i)
+		i=skipWhitespace(s,i+1)
+		return (i,res)
 
 	## Expand multiple games into distinct GameTrees and yield each.
 	def listGames(self):