diff --git a/src/compiler/parser.ts b/src/compiler/parser.ts
index 5b8504eb6dc..3209f234f85 100644
--- a/src/compiler/parser.ts
+++ b/src/compiler/parser.ts
@@ -2986,7 +2986,7 @@ namespace ts {
                 return parseFunctionOrConstructorType(SyntaxKind.ConstructorType);
             }
             const type = parseUnionTypeOrHigher();
-            if (!noConditionalTypes && parseOptional(SyntaxKind.ExtendsKeyword)) {
+            if (!noConditionalTypes && !scanner.hasPrecedingLineBreak() && parseOptional(SyntaxKind.ExtendsKeyword)) {
                 const node = <ConditionalTypeNode>createNode(SyntaxKind.ConditionalType, type.pos);
                 node.checkType = type;
                 // The type following 'extends' is not permitted to be another conditional type
diff --git a/tests/baselines/reference/conditionalTypesASI.js b/tests/baselines/reference/conditionalTypesASI.js
new file mode 100644
index 00000000000..0399d30f153
--- /dev/null
+++ b/tests/baselines/reference/conditionalTypesASI.js
@@ -0,0 +1,18 @@
+//// [conditionalTypesASI.ts]
+// Repro from #21637
+
+interface JSONSchema4 {
+  a?: number
+  extends?: string | string[]
+}
+
+
+//// [conditionalTypesASI.js]
+// Repro from #21637
+
+
+//// [conditionalTypesASI.d.ts]
+interface JSONSchema4 {
+    a?: number;
+    extends?: string | string[];
+}
diff --git a/tests/baselines/reference/conditionalTypesASI.symbols b/tests/baselines/reference/conditionalTypesASI.symbols
new file mode 100644
index 00000000000..00480cba82b
--- /dev/null
+++ b/tests/baselines/reference/conditionalTypesASI.symbols
@@ -0,0 +1,13 @@
+=== tests/cases/compiler/conditionalTypesASI.ts ===
+// Repro from #21637
+
+interface JSONSchema4 {
+>JSONSchema4 : Symbol(JSONSchema4, Decl(conditionalTypesASI.ts, 0, 0))
+
+  a?: number
+>a : Symbol(JSONSchema4.a, Decl(conditionalTypesASI.ts, 2, 23))
+
+  extends?: string | string[]
+>extends : Symbol(JSONSchema4.extends, Decl(conditionalTypesASI.ts, 3, 12))
+}
+
diff --git a/tests/baselines/reference/conditionalTypesASI.types b/tests/baselines/reference/conditionalTypesASI.types
new file mode 100644
index 00000000000..b0ee4502cfc
--- /dev/null
+++ b/tests/baselines/reference/conditionalTypesASI.types
@@ -0,0 +1,13 @@
+=== tests/cases/compiler/conditionalTypesASI.ts ===
+// Repro from #21637
+
+interface JSONSchema4 {
+>JSONSchema4 : JSONSchema4
+
+  a?: number
+>a : number
+
+  extends?: string | string[]
+>extends : string | string[]
+}
+
diff --git a/tests/cases/compiler/conditionalTypesASI.ts b/tests/cases/compiler/conditionalTypesASI.ts
new file mode 100644
index 00000000000..44d6d816851
--- /dev/null
+++ b/tests/cases/compiler/conditionalTypesASI.ts
@@ -0,0 +1,8 @@
+// @declaration: true
+
+// Repro from #21637
+
+interface JSONSchema4 {
+  a?: number
+  extends?: string | string[]
+}