diff --git a/lang/funcs/collection.go b/lang/funcs/collection.go
index d72d7261b..3ec505b00 100644
--- a/lang/funcs/collection.go
+++ b/lang/funcs/collection.go
@@ -441,8 +441,11 @@ var FlattenFunc = function.New(&function.Spec{
 			return cty.NilType, fmt.Errorf("can only flatten lists, sets and tuples")
 		}
 
-		outputList := make([]cty.Value, 0)
-		retVal := flattener(outputList, args[0])
+		retVal, known := flattener(args[0])
+		if !known {
+			return cty.DynamicPseudoType, nil
+		}
+
 		tys := make([]cty.Type, len(retVal))
 		for i, ty := range retVal {
 			tys[i] = ty.Type()
@@ -451,30 +454,41 @@ var FlattenFunc = function.New(&function.Spec{
 	},
 	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
 		inputList := args[0]
-		if !inputList.IsWhollyKnown() {
-			return cty.UnknownVal(retType), nil
-		}
 		if inputList.LengthInt() == 0 {
 			return cty.EmptyTupleVal, nil
 		}
-		outputList := make([]cty.Value, 0)
 
-		return cty.TupleVal(flattener(outputList, inputList)), nil
+		out, known := flattener(inputList)
+		if !known {
+			return cty.UnknownVal(retType), nil
+		}
+
+		return cty.TupleVal(out), nil
 	},
 })
 
-// Flatten until it's not a cty.List
-func flattener(finalList []cty.Value, flattenList cty.Value) []cty.Value {
+// Flatten until it's not a cty.List, and return whether the value is known.
+// We can flatten lists with unknown values, as long as they are not
+// lists themselves.
+func flattener(flattenList cty.Value) ([]cty.Value, bool) {
+	out := make([]cty.Value, 0)
 	for it := flattenList.ElementIterator(); it.Next(); {
 		_, val := it.Element()
-
 		if val.Type().IsListType() || val.Type().IsSetType() || val.Type().IsTupleType() {
-			finalList = flattener(finalList, val)
+			if !val.IsKnown() {
+				return out, false
+			}
+
+			res, known := flattener(val)
+			if !known {
+				return res, known
+			}
+			out = append(out, res...)
 		} else {
-			finalList = append(finalList, val)
+			out = append(out, val)
 		}
 	}
-	return finalList
+	return out, true
 }
 
 // KeysFunc contructs a function that takes a map and returns a sorted list of the map keys.
diff --git a/lang/funcs/collection_test.go b/lang/funcs/collection_test.go
index eadc3cddb..e6dbb04fa 100644
--- a/lang/funcs/collection_test.go
+++ b/lang/funcs/collection_test.go
@@ -1043,6 +1043,44 @@ func TestFlatten(t *testing.T) {
 			}),
 			false,
 		},
+		// handle single elements as arguments
+		{
+			cty.TupleVal([]cty.Value{
+				cty.ListVal([]cty.Value{
+					cty.StringVal("a"),
+					cty.StringVal("b"),
+				}),
+				cty.StringVal("c"),
+			}),
+			cty.TupleVal([]cty.Value{
+				cty.StringVal("a"),
+				cty.StringVal("b"),
+				cty.StringVal("c"),
+			}), false,
+		},
+		// handle single elements and mixed primitive types as arguments
+		{
+			cty.TupleVal([]cty.Value{
+				cty.ListVal([]cty.Value{
+					cty.StringVal("a"),
+					cty.StringVal("b"),
+				}),
+				cty.StringVal("c"),
+				cty.TupleVal([]cty.Value{
+					cty.StringVal("x"),
+					cty.NumberIntVal(1),
+				}),
+			}),
+			cty.TupleVal([]cty.Value{
+				cty.StringVal("a"),
+				cty.StringVal("b"),
+				cty.StringVal("c"),
+				cty.StringVal("x"),
+				cty.NumberIntVal(1),
+			}),
+			false,
+		},
+		// Primitive unknowns should still be flattened to a tuple
 		{
 			cty.ListVal([]cty.Value{
 				cty.ListVal([]cty.Value{
@@ -1054,8 +1092,26 @@ func TestFlatten(t *testing.T) {
 					cty.StringVal("d"),
 				}),
 			}),
-			cty.DynamicVal,
-			false,
+			cty.TupleVal([]cty.Value{
+				cty.StringVal("a"),
+				cty.StringVal("b"),
+				cty.UnknownVal(cty.String),
+				cty.StringVal("d"),
+			}), false,
+		},
+		// An unknown series should return an unknown dynamic value
+		{
+			cty.TupleVal([]cty.Value{
+				cty.ListVal([]cty.Value{
+					cty.StringVal("a"),
+					cty.StringVal("b"),
+				}),
+				cty.TupleVal([]cty.Value{
+					cty.UnknownVal(cty.List(cty.String)),
+					cty.StringVal("d"),
+				}),
+			}),
+			cty.UnknownVal(cty.DynamicPseudoType), false,
 		},
 		{
 			cty.ListValEmpty(cty.String),
@@ -1102,8 +1158,8 @@ func TestFlatten(t *testing.T) {
 		},
 	}
 
-	for _, test := range tests {
-		t.Run(fmt.Sprintf("flatten(%#v)", test.List), func(t *testing.T) {
+	for i, test := range tests {
+		t.Run(fmt.Sprintf("%d-flatten(%#v)", i, test.List), func(t *testing.T) {
 			got, err := Flatten(test.List)
 
 			if test.Err {