diff --git a/src/pkg/go/printer/nodes.go b/src/pkg/go/printer/nodes.go
index 6eee9a9245c..e99a2e36d4f 100644
--- a/src/pkg/go/printer/nodes.go
+++ b/src/pkg/go/printer/nodes.go
@@ -203,7 +203,7 @@ func (p *printer) exprList(prev0 token.Pos, list []ast.Expr, depth int, mode exp
 			} else {
 				const r = 4 // threshold
 				ratio := float64(size) / float64(prevSize)
-				useFF = ratio <= 1/r || r <= ratio
+				useFF = ratio <= 1.0/r || r <= ratio
 			}
 		}
 
diff --git a/src/pkg/go/printer/testdata/declarations.golden b/src/pkg/go/printer/testdata/declarations.golden
index 71ed32ed145..21bbf2b2d42 100644
--- a/src/pkg/go/printer/testdata/declarations.golden
+++ b/src/pkg/go/printer/testdata/declarations.golden
@@ -654,6 +654,35 @@ var _ = map[int]int{
 	abcde:	a,	// align with previous line
 }
 
+// alignment of map composite entries: test cases from issue 3965
+// aligned
+var _ = T1{
+	a:			x,
+	b:			y,
+	cccccccccccccccccccc:	z,
+}
+
+// not aligned
+var _ = T2{
+	a:	x,
+	b:	y,
+	ccccccccccccccccccccc:	z,
+}
+
+// aligned
+var _ = T3{
+	aaaaaaaaaaaaaaaaaaaa:	x,
+	b:			y,
+	c:			z,
+}
+
+// not aligned
+var _ = T4{
+	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa:	x,
+	b:	y,
+	c:	z,
+}
+
 func _() {
 	var _ = T{
 		a,	// must introduce trailing comma
diff --git a/src/pkg/go/printer/testdata/declarations.input b/src/pkg/go/printer/testdata/declarations.input
index d74cff25d1f..6ac00369926 100644
--- a/src/pkg/go/printer/testdata/declarations.input
+++ b/src/pkg/go/printer/testdata/declarations.input
@@ -667,6 +667,35 @@ var _ = map[int]int{
 	abcde: a, // align with previous line
 }
 
+// alignment of map composite entries: test cases from issue 3965
+// aligned
+var _ = T1{
+	a:                    x,
+	b:                    y,
+	cccccccccccccccccccc: z,
+}
+
+// not aligned
+var _ = T2{
+	a: x,
+	b: y,
+	ccccccccccccccccccccc: z,
+}
+
+// aligned
+var _ = T3{
+	aaaaaaaaaaaaaaaaaaaa: x,
+	b:                    y,
+	c:                    z,
+}
+
+// not aligned
+var _ = T4{
+	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa: x,
+	b:                                       y,
+	c:                                       z,
+}
+
 
 func _() {
 	var _ = T{