diff --git a/lang/funcs/collection.go b/lang/funcs/collection.go
index bc516f2c3..71b7a8466 100644
--- a/lang/funcs/collection.go
+++ b/lang/funcs/collection.go
@@ -1064,11 +1064,11 @@ var SliceFunc = function.New(&function.Spec{
 			Type: cty.DynamicPseudoType,
 		},
 		{
-			Name: "startIndex",
+			Name: "start_index",
 			Type: cty.Number,
 		},
 		{
-			Name: "endIndex",
+			Name: "end_index",
 			Type: cty.Number,
 		},
 	},
@@ -1076,25 +1076,39 @@ var SliceFunc = function.New(&function.Spec{
 		arg := args[0]
 		argTy := arg.Type()
 
+		if argTy.IsSetType() {
+			return cty.NilType, function.NewArgErrorf(0, "cannot slice a set, because its elements do not have indices; use the tolist function to force conversion to list if the ordering of the result is not important")
+		}
 		if !argTy.IsListType() && !argTy.IsTupleType() {
-			return cty.NilType, errors.New("cannot slice a set, because its elements do not have indices; use the tolist function to force conversion to list if the ordering of the result is not important")
+			return cty.NilType, function.NewArgErrorf(0, "must be a list or tuple value")
+		}
+
+		startIndex, endIndex, idxsKnown, err := sliceIndexes(args)
+		if err != nil {
+			return cty.NilType, err
 		}
 
 		if argTy.IsListType() {
 			return argTy, nil
 		}
 
-		startIndex, endIndex, err := sliceIndexes(args, args[0].LengthInt())
-		if err != nil {
-			return cty.NilType, err
+		if !idxsKnown {
+			// If we don't know our start/end indices then we can't predict
+			// the result type if we're planning to return a tuple.
+			return cty.DynamicPseudoType, nil
 		}
-
 		return cty.Tuple(argTy.TupleElementTypes()[startIndex:endIndex]), nil
 	},
 	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
 		inputList := args[0]
 
-		startIndex, endIndex, err := sliceIndexes(args, inputList.LengthInt())
+		if retType == cty.DynamicPseudoType {
+			return cty.DynamicVal, nil
+		}
+
+		// we ignore idxsKnown return value here because the indices are always
+		// known here, or else the call would've short-circuited.
+		startIndex, endIndex, _, err := sliceIndexes(args)
 		if err != nil {
 			return cty.NilVal, err
 		}
@@ -1116,26 +1130,45 @@ var SliceFunc = function.New(&function.Spec{
 	},
 })
 
-func sliceIndexes(args []cty.Value, max int) (int, int, error) {
-	var startIndex, endIndex int
+func sliceIndexes(args []cty.Value) (int, int, bool, error) {
+	var startIndex, endIndex, length int
+	var startKnown, endKnown, lengthKnown bool
 
-	if err := gocty.FromCtyValue(args[1], &startIndex); err != nil {
-		return 0, 0, fmt.Errorf("invalid start index: %s", err)
-	}
-	if err := gocty.FromCtyValue(args[2], &endIndex); err != nil {
-		return 0, 0, fmt.Errorf("invalid start index: %s", err)
+	if args[0].Type().IsTupleType() || args[0].IsKnown() { // if it's a tuple then we always know the length by the type, but lists must be known
+		length = args[0].LengthInt()
+		lengthKnown = true
 	}
 
-	if startIndex < 0 {
-		return 0, 0, errors.New("from index must be greater than or equal to 0")
+	if args[1].IsKnown() {
+		if err := gocty.FromCtyValue(args[1], &startIndex); err != nil {
+			return 0, 0, false, function.NewArgErrorf(1, "invalid start index: %s", err)
+		}
+		if startIndex < 0 {
+			return 0, 0, false, function.NewArgErrorf(1, "start index must not be less than zero")
+		}
+		if lengthKnown && startIndex > length {
+			return 0, 0, false, function.NewArgErrorf(1, "start index must not be greater than the length of the list")
+		}
+		startKnown = true
 	}
-	if endIndex > max {
-		return 0, 0, errors.New("to index must be less than or equal to the length of the input list")
+	if args[2].IsKnown() {
+		if err := gocty.FromCtyValue(args[2], &endIndex); err != nil {
+			return 0, 0, false, function.NewArgErrorf(2, "invalid end index: %s", err)
+		}
+		if endIndex < 0 {
+			return 0, 0, false, function.NewArgErrorf(2, "end index must not be less than zero")
+		}
+		if lengthKnown && endIndex > length {
+			return 0, 0, false, function.NewArgErrorf(2, "end index must not be greater than the length of the list")
+		}
+		endKnown = true
 	}
-	if startIndex > endIndex {
-		return 0, 0, errors.New("from index must be less than or equal to index")
+	if startKnown && endKnown {
+		if startIndex > endIndex {
+			return 0, 0, false, function.NewArgErrorf(1, "start index must not be greater than end index")
+		}
 	}
-	return startIndex, endIndex, nil
+	return startIndex, endIndex, startKnown && endKnown, nil
 }
 
 // TransposeFunc contructs a function that takes a map of lists of strings and
diff --git a/lang/funcs/collection_test.go b/lang/funcs/collection_test.go
index dd112388f..d1a543797 100644
--- a/lang/funcs/collection_test.go
+++ b/lang/funcs/collection_test.go
@@ -2595,6 +2595,16 @@ func TestSlice(t *testing.T) {
 			}),
 			false,
 		},
+		{ // unknown tuple slice
+			cty.UnknownVal(tuple.Type()),
+			cty.NumberIntVal(1),
+			cty.NumberIntVal(3),
+			cty.UnknownVal(cty.Tuple([]cty.Type{
+				cty.Number,
+				cty.List(cty.String),
+			})),
+			false,
+		},
 		{ // empty list slice
 			listOfStrings,
 			cty.NumberIntVal(2),
@@ -2609,6 +2619,90 @@ func TestSlice(t *testing.T) {
 			cty.EmptyTupleVal,
 			false,
 		},
+		{ // list with unknown start offset
+			listOfStrings,
+			cty.UnknownVal(cty.Number),
+			cty.NumberIntVal(2),
+			cty.UnknownVal(cty.List(cty.String)),
+			false,
+		},
+		{ // list with unknown start offset but end out of bounds
+			listOfStrings,
+			cty.UnknownVal(cty.Number),
+			cty.NumberIntVal(200),
+			cty.UnknownVal(cty.List(cty.String)),
+			true,
+		},
+		{ // list with unknown start offset but end < 0
+			listOfStrings,
+			cty.UnknownVal(cty.Number),
+			cty.NumberIntVal(-4),
+			cty.UnknownVal(cty.List(cty.String)),
+			true,
+		},
+		{ // list with unknown end offset
+			listOfStrings,
+			cty.UnknownVal(cty.Number),
+			cty.NumberIntVal(0),
+			cty.UnknownVal(cty.List(cty.String)),
+			false,
+		},
+		{ // list with unknown end offset but start out of bounds
+			listOfStrings,
+			cty.UnknownVal(cty.Number),
+			cty.NumberIntVal(200),
+			cty.UnknownVal(cty.List(cty.String)),
+			true,
+		},
+		{ // list with unknown end offset but start < 0
+			listOfStrings,
+			cty.UnknownVal(cty.Number),
+			cty.NumberIntVal(-3),
+			cty.UnknownVal(cty.List(cty.String)),
+			true,
+		},
+		{ // tuple slice with unknown start offset
+			tuple,
+			cty.UnknownVal(cty.Number),
+			cty.NumberIntVal(3),
+			cty.DynamicVal,
+			false,
+		},
+		{ // tuple slice with unknown start offset but end out of bounds
+			tuple,
+			cty.UnknownVal(cty.Number),
+			cty.NumberIntVal(200),
+			cty.DynamicVal,
+			true,
+		},
+		{ // tuple slice with unknown start offset but end < 0
+			tuple,
+			cty.UnknownVal(cty.Number),
+			cty.NumberIntVal(-20),
+			cty.DynamicVal,
+			true,
+		},
+		{ // tuple slice with unknown end offset
+			tuple,
+			cty.NumberIntVal(0),
+			cty.UnknownVal(cty.Number),
+			cty.DynamicVal,
+			false,
+		},
+		{ // tuple slice with unknown end offset but start < 0
+			tuple,
+			cty.NumberIntVal(-2),
+			cty.UnknownVal(cty.Number),
+			cty.DynamicVal,
+			true,
+		},
+		{ // tuple slice with unknown end offset but start out of bounds
+			tuple,
+			cty.NumberIntVal(200),
+			cty.UnknownVal(cty.Number),
+			cty.DynamicVal,
+			true,
+		},
 	}
 
 	for i, test := range tests {