diff --git a/ast/parser_ext.go b/ast/parser_ext.go
index d20bb8a51c..9151fa378c 100644
--- a/ast/parser_ext.go
+++ b/ast/parser_ext.go
@@ -142,6 +142,10 @@ func ParseRuleFromExpr(module *Module, expr *Expr) (*Rule, error) {
 		return nil, fmt.Errorf("negated expressions cannot be used for rule head")
 	}
 
+	if _, ok := expr.Terms.(*SomeDecl); ok {
+		return nil, errors.New("some declarations cannot be used for rule head")
+	}
+
 	if term, ok := expr.Terms.(*Term); ok {
 		switch v := term.Value.(type) {
 		case Ref:
@@ -151,6 +155,12 @@ func ParseRuleFromExpr(module *Module, expr *Expr) (*Rule, error) {
 		}
 	}
 
+	if _, ok := expr.Terms.([]*Term); !ok {
+		// This is a defensive check in case other kinds of expression terms are
+		// introduced in the future.
+		return nil, errors.New("expression cannot be used for rule head")
+	}
+
 	if expr.IsAssignment() {
 
 		lhs, rhs := expr.Operand(0), expr.Operand(1)
diff --git a/ast/parser_test.go b/ast/parser_test.go
index 4b83c90ae7..251f1cb170 100644
--- a/ast/parser_test.go
+++ b/ast/parser_test.go
@@ -1427,6 +1427,11 @@ data = {"bar": 2} { true }`
 
 	"foo" := 1`
 
+	someDecl := `
+	package a
+
+	some x`
+
 	assertParseModuleError(t, "multiple expressions", multipleExprs)
 	assertParseModuleError(t, "non-equality", nonEquality)
 	assertParseModuleError(t, "non-var name", nonVarName)
@@ -1437,6 +1442,13 @@ data = {"bar": 2} { true }`
 	assertParseModuleError(t, "non ref term", nonRefTerm)
 	assertParseModuleError(t, "zero args", zeroArgs)
 	assertParseModuleError(t, "assign to term", assignToTerm)
+	assertParseModuleError(t, "some decl", someDecl)
+
+	if _, err := ParseRuleFromExpr(&Module{}, &Expr{
+		Terms: struct{}{},
+	}); err == nil {
+		t.Fatal("expected error for unknown expression term type")
+	}
 }
 
 func TestWildcards(t *testing.T) {