diff --git a/script/test.py b/script/test.py
index 023e9534..dcd83756 100755
--- a/script/test.py
+++ b/script/test.py
@@ -19,7 +19,7 @@ EXPECT_ERROR_PATTERN = re.compile(r'// expect error(?! line)')
 EXPECT_ERROR_LINE_PATTERN = re.compile(r'// expect error line (\d+)')
 EXPECT_RUNTIME_ERROR_PATTERN = re.compile(r'// expect runtime error: (.+)')
 ERROR_PATTERN = re.compile(r'\[.* line (\d+)\] Error')
-STACK_TRACE_PATTERN = re.compile(r'\[.* line (\d+)\] in')
+STACK_TRACE_PATTERN = re.compile(r'\[main line (\d+)\] in')
 STDIN_PATTERN = re.compile(r'// stdin: (.*)')
 SKIP_PATTERN = re.compile(r'// skip: (.*)')
 NONTEST_PATTERN = re.compile(r'// nontest')
@@ -159,11 +159,17 @@ class Test:
           self.runtime_error_message)
       self.fail(error_lines[0])
 
-    # Make sure the stack trace has the right line.
-    match = STACK_TRACE_PATTERN.search(error_lines[1])
+    # Make sure the stack trace has the right line. Skip over any lines that
+    # come from builtin libraries.
+    stack_lines = error_lines[1:]
+    for stack_line in stack_lines:
+      match = STACK_TRACE_PATTERN.search(stack_line)
+      if match: break
+
     if not match:
       self.fail('Expected stack trace and got:')
-      self.fail(error_lines[1])
+      for stack_line in stack_lines:
+        self.fail(stack_line)
     else:
       stack_line = int(match.group(1))
       if stack_line != self.runtime_error_line:
diff --git a/src/module/timer.c b/src/module/timer.c
index 62c3eb48..9cfba26f 100644
--- a/src/module/timer.c
+++ b/src/module/timer.c
@@ -11,6 +11,8 @@
 static const char* timerLibSource =
 "class Timer {\n"
 "  static sleep(milliseconds) {\n"
+"    if (!(milliseconds is Num)) Fiber.abort(\"Milliseconds must be a number.\")\n"
+"    if (milliseconds < 0) Fiber.abort(\"Milliseconds cannot be negative.\")\n"
 "    startTimer_(milliseconds, Fiber.current)\n"
 "    Fiber.yield()\n"
 "  }\n"
diff --git a/src/module/timer.h b/src/module/timer.h
index 0f8f311d..8c3384b9 100644
--- a/src/module/timer.h
+++ b/src/module/timer.h
@@ -3,7 +3,6 @@
 
 #include "wren.h"
 
-// TODO: Coherent naming scheme.
 char* timerGetSource();
 
 WrenForeignMethodFn timerBindForeign(
diff --git a/src/module/timer.wren b/src/module/timer.wren
index 18f54eaf..a6ea328d 100644
--- a/src/module/timer.wren
+++ b/src/module/timer.wren
@@ -1,5 +1,7 @@
 class Timer {
   static sleep(milliseconds) {
+    if (!(milliseconds is Num)) Fiber.abort("Milliseconds must be a number.")
+    if (milliseconds < 0) Fiber.abort("Milliseconds cannot be negative.")
     startTimer_(milliseconds, Fiber.current)
     Fiber.yield()
   }
diff --git a/test/timer/sleep_fibers.wren b/test/timer/sleep_fibers.wren
new file mode 100644
index 00000000..88e308c9
--- /dev/null
+++ b/test/timer/sleep_fibers.wren
@@ -0,0 +1,19 @@
+import "timer" for Timer
+
+Fiber.new {
+  Timer.sleep(2)
+  IO.print("a")
+}.call()
+
+Fiber.new {
+  Timer.sleep(1)
+  IO.print("b")
+}.call()
+
+IO.print("main")
+Timer.sleep(3)
+IO.print("done")
+// expect: main
+// expect: b
+// expect: a
+// expect: done
diff --git a/test/timer/sleep_float.wren b/test/timer/sleep_float.wren
new file mode 100644
index 00000000..f04b6111
--- /dev/null
+++ b/test/timer/sleep_float.wren
@@ -0,0 +1,20 @@
+import "timer" for Timer
+
+// These are both rounded to 1, so "a" should complete first.
+Fiber.new {
+  Timer.sleep(1.5)
+  IO.print("a")
+}.call()
+
+Fiber.new {
+  Timer.sleep(1.3)
+  IO.print("b")
+}.call()
+
+IO.print("main")
+Timer.sleep(3)
+IO.print("done")
+// expect: main
+// expect: a
+// expect: b
+// expect: done
diff --git a/test/timer/sleep_main_fiber.wren b/test/timer/sleep_main_fiber.wren
new file mode 100644
index 00000000..3e3e43fb
--- /dev/null
+++ b/test/timer/sleep_main_fiber.wren
@@ -0,0 +1,9 @@
+import "timer" for Timer
+
+IO.print("1") // expect: 1
+Timer.sleep(3)
+IO.print("2") // expect: 2
+Timer.sleep(3)
+IO.print("3") // expect: 3
+Timer.sleep(3)
+IO.print("4") // expect: 4
diff --git a/test/timer/sleep_negative.wren b/test/timer/sleep_negative.wren
new file mode 100644
index 00000000..15afb714
--- /dev/null
+++ b/test/timer/sleep_negative.wren
@@ -0,0 +1,3 @@
+import "timer" for Timer
+
+Timer.sleep(-1) // expect runtime error: Milliseconds cannot be negative.
diff --git a/test/timer/sleep_not_num.wren b/test/timer/sleep_not_num.wren
new file mode 100644
index 00000000..cbd3d1ed
--- /dev/null
+++ b/test/timer/sleep_not_num.wren
@@ -0,0 +1,3 @@
+import "timer" for Timer
+
+Timer.sleep("wat") // expect runtime error: Milliseconds must be a number.
diff --git a/test/timer/sleep_zero.wren b/test/timer/sleep_zero.wren
new file mode 100644
index 00000000..569ffe13
--- /dev/null
+++ b/test/timer/sleep_zero.wren
@@ -0,0 +1,27 @@
+import "timer" for Timer
+
+Fiber.new {
+  Timer.sleep(0)
+  IO.print("a")
+}.call()
+
+Fiber.new {
+  Timer.sleep(0)
+  IO.print("b")
+}.call()
+
+Fiber.new {
+  Timer.sleep(0)
+  IO.print("c")
+}.call()
+
+IO.print("main")
+Timer.sleep(0) // This is enough to let the other fiber run.
+IO.print("done")
+
+// expect: main
+// Run in the order they were enqueued.
+// expect: a
+// expect: b
+// expect: c
+// expect: done