diff --git a/src/public_interface.jl b/src/public_interface.jl
index fe0d033..1f73f51 100644
--- a/src/public_interface.jl
+++ b/src/public_interface.jl
@@ -14,7 +14,7 @@ function start(path::AbstractString)
     if isfile(path)
         return _start_tracking_file(path)
     else
-        throw(ArgumentError("Path must be a file"))
+        throw(ArgumentError("Path must be an existing file"))
     end
 end
 
@@ -37,12 +37,11 @@ function stop(; dump_coverage::Bool = false,
 end
 
 function clean(path::AbstractString = pwd())
-    _realpath::String = convert(String, realpath(path))::String
-    if isfile(_realpath)
-        return _clean_file(_realpath)
-    elseif isdir(_realpath)
-        return _clean_directory(_realpath)
+    if isfile(path)
+        return _clean_file(convert(String, realpath(path)))
+    elseif isdir(path)
+        return _clean_directory(convert(String, realpath(path)))
     else
-        throw(ArgumentError("Path must be a file or directory"))
+        throw(ArgumentError("Path must be an existing file or directory"))
     end
 end
diff --git a/test/runtests.jl b/test/runtests.jl
index d857c48..24aae67 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -11,9 +11,14 @@ using Test
         @test MicroCoverage.always_assert(true, "") == nothing
         @test_throws MicroCoverage.AlwaysAssertionError MicroCoverage.always_assert(false, "")
     end
+    @testset "instrument.jl" begin
+        MicroCoverage.instrument("", Expr(:block), Val(:block)) == Expr(:block)
+    end
     @testset "public_interface.jl" begin
         MicroCoverage.with_temp_dir() do tmp_depot
             MicroCoverage.with_temp_dir() do tmp_src_directory
+                nonexistent_filename = joinpath(tmp_src_directory,
+                                                "nonexistent.jl")
                 foo_jl_filename = joinpath(tmp_src_directory,
                                            "foo.jl")
                 test_foo_jl_filename = joinpath(tmp_src_directory,
@@ -34,7 +39,9 @@ using Test
                     println(io, "")
                     println(io, "@test foo(1) == \"hello\"")
                 end
-                MicroCoverage.start(foo_jl_filename)
+                @test_throws ArgumentError MicroCoverage.start(nonexistent_filename)
+                @test_throws ArgumentError MicroCoverage.clean(nonexistent_filename)
+                MicroCoverage.start(strip(foo_jl_filename))
                 include(foo_jl_filename)
                 MicroCoverage.preview_coverage(; dump_coverage_io = devnull)
                 include(test_foo_jl_filename)
@@ -47,7 +54,9 @@ using Test
                 @test isfile(foo_jl_microcov_filename)
                 @test ispath(foo_jl_microcov_filename)
                 foo_jl_microcov_filecontents = read(foo_jl_microcov_filename, String)
+                touch(foo_jl_microcov_filename)
                 MicroCoverage.clean(foo_jl_filename)
+                touch(foo_jl_microcov_filename)
                 MicroCoverage.clean(tmp_src_directory)
                 @test foo_jl_microcov_filecontents == string("[1,1,1]                 function foo(x)\n",
                                                              "[1,0,1,0,1]                 if x == 1 || x == 100\n",