diff --git a/src/wren_core.c b/src/wren_core.c
index 0b9aba1d..4b0d9bd4 100644
--- a/src/wren_core.c
+++ b/src/wren_core.c
@@ -196,11 +196,18 @@ DEF_NATIVE(fiber_create)
   RETURN_VAL(OBJ_VAL(newFiber));
 }
 
+DEF_NATIVE(fiber_isDone)
+{
+  ObjFiber* runFiber = AS_FIBER(args[0]);
+  RETURN_BOOL(runFiber->numFrames == 0);
+}
+
 DEF_NATIVE(fiber_run)
 {
-  // TODO: Error if the fiber is already complete.
   ObjFiber* runFiber = AS_FIBER(args[0]);
 
+  if (runFiber->numFrames == 0) RETURN_ERROR("Cannot run a finished fiber.");
+
   // Remember who ran it.
   runFiber->caller = fiber;
 
@@ -215,9 +222,10 @@ DEF_NATIVE(fiber_run)
 
 DEF_NATIVE(fiber_run1)
 {
-  // TODO: Error if the fiber is already complete.
   ObjFiber* runFiber = AS_FIBER(args[0]);
 
+  if (runFiber->numFrames == 0) RETURN_ERROR("Cannot run a finished fiber.");
+
   // Remember who ran it.
   runFiber->caller = fiber;
 
@@ -239,7 +247,7 @@ DEF_NATIVE(fiber_run1)
 
 DEF_NATIVE(fiber_yield)
 {
-  // TODO: Handle caller being null.
+  if (fiber->caller == NULL) RETURN_ERROR("No fiber to yield to.");
 
   // Make the caller's run method return null.
   fiber->caller->stack[fiber->caller->stackSize - 1] = NULL_VAL;
@@ -251,7 +259,7 @@ DEF_NATIVE(fiber_yield)
 
 DEF_NATIVE(fiber_yield1)
 {
-  // TODO: Handle caller being null.
+  if (fiber->caller == NULL) RETURN_ERROR("No fiber to yield to.");
 
   // Make the caller's run method return the argument passed to yield.
   fiber->caller->stack[fiber->caller->stackSize - 1] = args[1];
@@ -698,6 +706,7 @@ void wrenInitializeCore(WrenVM* vm)
   NATIVE(vm->fiberClass->metaclass, "create ", fiber_create);
   NATIVE(vm->fiberClass->metaclass, "yield", fiber_yield);
   NATIVE(vm->fiberClass->metaclass, "yield ", fiber_yield1);
+  NATIVE(vm->fiberClass, "isDone", fiber_isDone);
   NATIVE(vm->fiberClass, "run", fiber_run);
   NATIVE(vm->fiberClass, "run ", fiber_run1);
   // TODO: Primitives for switching to a fiber without setting the caller.
diff --git a/test/fiber/is_done.wren b/test/fiber/is_done.wren
new file mode 100644
index 00000000..49fd5a18
--- /dev/null
+++ b/test/fiber/is_done.wren
@@ -0,0 +1,11 @@
+var fiber = Fiber.create(fn {
+  IO.print("1")
+  Fiber.yield
+  IO.print("2")
+})
+
+IO.print(fiber.isDone)  // expect: false
+fiber.run               // expect: 1
+IO.print(fiber.isDone)  // expect: false
+fiber.run               // expect: 2
+IO.print(fiber.isDone)  // expect: true
diff --git a/test/fiber/run_when_done.wren b/test/fiber/run_when_done.wren
new file mode 100644
index 00000000..7282f1e0
--- /dev/null
+++ b/test/fiber/run_when_done.wren
@@ -0,0 +1,6 @@
+var fiber = Fiber.create(fn {
+  IO.print("run")
+})
+
+fiber.run // expect: run
+fiber.run // expect runtime error: Cannot run a finished fiber.
diff --git a/test/fiber/run_with_value_when_done.wren b/test/fiber/run_with_value_when_done.wren
new file mode 100644
index 00000000..09542b40
--- /dev/null
+++ b/test/fiber/run_with_value_when_done.wren
@@ -0,0 +1,6 @@
+var fiber = Fiber.create(fn {
+  IO.print("run")
+})
+
+fiber.run(1) // expect: run
+fiber.run(2) // expect runtime error: Cannot run a finished fiber.
diff --git a/test/fiber/yield_from_main.wren b/test/fiber/yield_from_main.wren
new file mode 100644
index 00000000..b887c2ea
--- /dev/null
+++ b/test/fiber/yield_from_main.wren
@@ -0,0 +1 @@
+Fiber.yield // expect runtime error: No fiber to yield to.
diff --git a/test/fiber/yield_with_value_from_main.wren b/test/fiber/yield_with_value_from_main.wren
new file mode 100644
index 00000000..0b890db0
--- /dev/null
+++ b/test/fiber/yield_with_value_from_main.wren
@@ -0,0 +1 @@
+Fiber.yield(1) // expect runtime error: No fiber to yield to.