diff --git a/node.go b/node.go
index 8315f77..6e007f3 100644
--- a/node.go
+++ b/node.go
@@ -42,6 +42,22 @@ func (pe parseErr) Unwrap() error {
 
 //
 
+type arityErr struct {
+	funcName                     string
+	got                          int
+	expected, expectedUpperRange int
+}
+
+func (ae arityErr) Error() string {
+	if ae.expected != ae.expectedUpperRange {
+		return fmt.Sprintf("Call to '%s' expected %d to %d argument(s), got %d", ae.funcName, ae.expected, ae.expectedUpperRange, ae.got)
+	} else {
+		return fmt.Sprintf("Call to '%s' expected %d argument(s), got %d", ae.funcName, ae.expected, ae.got)
+	}
+}
+
+//
+
 type ParseContext int
 
 const (
@@ -678,7 +694,7 @@ func (this *conversionState) convertNoFreeFloating(n_ node.Node) (string, error)
 				// define() gets converted to const
 
 				if len(fncall.ArgumentList.Arguments) != 2 {
-					return "", parseErr{fncall, fmt.Errorf("expected define() to have 2 arguments, found %d", len(fncall.ArgumentList.Arguments))}
+					return "", parseErr{fncall, arityErr{"define", len(fncall.ArgumentList.Arguments), 2, 2}}
 				}
 
 				defineName := fncall.ArgumentList.Arguments[0].(*node.Argument).Expr // that much is always possible
@@ -1115,7 +1131,7 @@ func (this *conversionState) convertNoFreeFloating(n_ node.Node) (string, error)
 
 		} else if funcName == `explode` {
 			if len(callParams) != 2 {
-				return "", parseErr{n, fmt.Errorf("Call to '%s' expected 2 argument(s), got %d", funcName, len(callParams))}
+				return "", parseErr{n, arityErr{funcName, len(callParams), 2, 2}}
 			}
 			this.importPackages["strings"] = struct{}{}
 			funcName = `strings.Split`
@@ -1123,7 +1139,7 @@ func (this *conversionState) convertNoFreeFloating(n_ node.Node) (string, error)
 
 		} else if funcName == `implode` {
 			if len(callParams) != 2 {
-				return "", parseErr{n, fmt.Errorf("Call to '%s' expected 2 argument(s), got %d", funcName, len(callParams))}
+				return "", parseErr{n, arityErr{funcName, len(callParams), 2, 2}}
 			}
 
 			this.importPackages["strings"] = struct{}{}
@@ -1136,7 +1152,7 @@ func (this *conversionState) convertNoFreeFloating(n_ node.Node) (string, error)
 
 		} else if funcName == `trim` {
 			if len(callParams) != 1 {
-				return "", parseErr{n, fmt.Errorf("Call to '%s' expected 2 argument(s), got %d", funcName, len(callParams))}
+				return "", parseErr{n, arityErr{funcName, len(callParams), 2, 2}}
 			}
 
 			this.importPackages["strings"] = struct{}{}
@@ -1144,7 +1160,7 @@ func (this *conversionState) convertNoFreeFloating(n_ node.Node) (string, error)
 
 		} else if funcName == `rtrim` {
 			if len(callParams) != 1 {
-				return "", parseErr{n, fmt.Errorf("Call to '%s' expected 1 argument(s), got %d", funcName, len(callParams))}
+				return "", parseErr{n, arityErr{funcName, len(callParams), 1, 1}}
 			}
 
 			this.importPackages["strings"] = struct{}{}
@@ -1154,7 +1170,7 @@ func (this *conversionState) convertNoFreeFloating(n_ node.Node) (string, error)
 
 		} else if funcName == `ltrim` {
 			if len(callParams) != 1 {
-				return "", parseErr{n, fmt.Errorf("Call to '%s' expected 1 argument(s), got %d", funcName, len(callParams))}
+				return "", parseErr{n, arityErr{funcName, len(callParams), 1, 1}}
 			}
 
 			this.importPackages["strings"] = struct{}{}
@@ -1164,7 +1180,7 @@ func (this *conversionState) convertNoFreeFloating(n_ node.Node) (string, error)
 
 		} else if funcName == `strtolower` {
 			if len(callParams) != 1 {
-				return "", parseErr{n, fmt.Errorf("Call to '%s' expected 2 argument(s), got %d", funcName, len(callParams))}
+				return "", parseErr{n, arityErr{funcName, len(callParams), 1, 1}}
 			}
 
 			this.importPackages["strings"] = struct{}{}
@@ -1172,7 +1188,7 @@ func (this *conversionState) convertNoFreeFloating(n_ node.Node) (string, error)
 
 		} else if funcName == `strtoupper` {
 			if len(callParams) != 1 {
-				return "", parseErr{n, fmt.Errorf("Call to '%s' expected 2 argument(s), got %d", funcName, len(callParams))}
+				return "", parseErr{n, arityErr{funcName, len(callParams), 1, 1}}
 			}
 
 			this.importPackages["strings"] = struct{}{}
@@ -1180,7 +1196,7 @@ func (this *conversionState) convertNoFreeFloating(n_ node.Node) (string, error)
 
 		} else if funcName == `file_put_contents` {
 			if len(callParams) != 2 {
-				return "", parseErr{n, fmt.Errorf("Call to '%s' expected 2 argument(s), got %d", funcName, len(callParams))}
+				return "", parseErr{n, arityErr{funcName, len(callParams), 2, 2}}
 			}
 
 			this.importPackages["io/ioutil"] = struct{}{}
@@ -1193,7 +1209,7 @@ func (this *conversionState) convertNoFreeFloating(n_ node.Node) (string, error)
 			// str_replace( mixed $search , mixed $replace , mixed $subject [, int &$count ] )
 			// n.b. str_replace also supports arrays, that strings.Replace() doesn't do
 			if !(len(callParams) == 3 || len(callParams) == 4) {
-				return "", parseErr{n, fmt.Errorf("Call to '%s' expected 3 or 4 argument(s), got %d", funcName, len(callParams))}
+				return "", parseErr{n, arityErr{funcName, len(callParams), 3, 4}}
 			}
 
 			this.importPackages["strings"] = struct{}{}
@@ -1240,7 +1256,7 @@ func (this *conversionState) convertNoFreeFloating(n_ node.Node) (string, error)
 				funcName = `fmt.Sprintf`
 
 			} else {
-				return "", parseErr{n, fmt.Errorf("Call to '%s' expected 3 or 4 argument(s), got %d", funcName, len(callParams))}
+				return "", parseErr{n, arityErr{funcName, len(callParams), 3, 4}}
 
 			}
 
@@ -1260,16 +1276,15 @@ func (this *conversionState) convertNoFreeFloating(n_ node.Node) (string, error)
 
 				// WARNING: We are doubling up on the first parameter
 				// This is a problem if it's not necessarily constant
-
 				return "(" + callParams[0] + ")[" + callParams[1] + ":" + callParams[1] + `+` + callParams[2] + "]", nil
 
 			} else {
-				return "", parseErr{n, fmt.Errorf("Call to '%s' expected 2 or 3 argument(s), got %d", funcName, len(callParams))}
+				return "", parseErr{n, arityErr{funcName, len(callParams), 2, 3}}
 			}
 
 		} else if funcName == `rename` {
 			if len(callParams) != 2 {
-				return "", parseErr{n, fmt.Errorf("Call to '%s' expected 2 argument(s), got %d", funcName, len(callParams))}
+				return "", parseErr{n, arityErr{funcName, len(callParams), 2, 2}}
 			}
 
 			this.importPackages["os"] = struct{}{}