diff --git a/spec/ameba/cli/cmd_spec.cr b/spec/ameba/cli/cmd_spec.cr
index a412c194..867b6d45 100644
--- a/spec/ameba/cli/cmd_spec.cr
+++ b/spec/ameba/cli/cmd_spec.cr
@@ -5,7 +5,7 @@ module Ameba::Cli
   describe "Cmd" do
     describe ".run" do
       it "runs ameba" do
-        r = Cli.run %w(-f silent file.cr)
+        r = Cli.run %w(-f silent -c spec/fixtures/config.yml spec/fixtures/source.cr)
         r.should be_nil
       end
     end
@@ -43,12 +43,12 @@ module Ameba::Cli
       end
 
       it "defaults rules? flag to false" do
-        c = Cli.parse_args %w(file.cr)
+        c = Cli.parse_args %w(spec/fixtures/source.cr)
         c.rules?.should be_false
       end
 
       it "defaults skip_reading_config? flag to false" do
-        c = Cli.parse_args %w(file.cr)
+        c = Cli.parse_args %w(spec/fixtures/source.cr)
         c.skip_reading_config?.should be_false
       end
 
@@ -58,7 +58,7 @@ module Ameba::Cli
       end
 
       it "defaults all? flag to false" do
-        c = Cli.parse_args %w(file.cr)
+        c = Cli.parse_args %w(spec/fixtures/source.cr)
         c.all?.should be_false
       end
 
@@ -95,35 +95,35 @@ module Ameba::Cli
 
       describe "-e/--explain" do
         it "configures file/line/column" do
-          c = Cli.parse_args %w(--explain src/file.cr:3:5)
+          c = Cli.parse_args %w(--explain spec/fixtures/source.cr:3:5)
 
           location_to_explain = c.location_to_explain.should_not be_nil
-          location_to_explain[:file].should eq "src/file.cr"
+          location_to_explain[:file].should eq "spec/fixtures/source.cr"
           location_to_explain[:line].should eq 3
           location_to_explain[:column].should eq 5
         end
 
         it "raises an error if location is not valid" do
           expect_raises(Exception, "location should have PATH:line:column") do
-            Cli.parse_args %w(--explain src/file.cr:3)
+            Cli.parse_args %w(--explain spec/fixtures/source.cr:3)
           end
         end
 
         it "raises an error if line number is not valid" do
           expect_raises(Exception, "location should have PATH:line:column") do
-            Cli.parse_args %w(--explain src/file.cr:a:3)
+            Cli.parse_args %w(--explain spec/fixtures/source.cr:a:3)
           end
         end
 
         it "raises an error if column number is not valid" do
           expect_raises(Exception, "location should have PATH:line:column") do
-            Cli.parse_args %w(--explain src/file.cr:3:&)
+            Cli.parse_args %w(--explain spec/fixtures/source.cr:3:&)
           end
         end
 
         it "raises an error if line/column are missing" do
           expect_raises(Exception, "location should have PATH:line:column") do
-            Cli.parse_args %w(--explain src/file.cr)
+            Cli.parse_args %w(--explain spec/fixtures/source.cr)
           end
         end
       end
diff --git a/spec/ameba/config_spec.cr b/spec/ameba/config_spec.cr
index 1c1d10b2..036c458f 100644
--- a/spec/ameba/config_spec.cr
+++ b/spec/ameba/config_spec.cr
@@ -9,10 +9,21 @@ module Ameba
     end
 
     describe ".new" do
-      it "loads default globs when config is empty" do
-        yml = YAML.parse "{}"
-        config = Config.new(yml)
-        config.globs.should eq Config::DEFAULT_GLOBS
+      context "when config is empty" do
+        it "loads default globs" do
+          yml = YAML.parse "{}"
+          config = Config.new(yml)
+          config.globs.should eq ["**/*.cr"]
+        end
+
+        it "sets !lib as a default glob when there are .cr files in lib" do
+          File.touch "lib/shard.cr"
+          yml = YAML.parse "{}"
+          config = Config.new(yml)
+          config.globs.should eq ["**/*.cr", "!lib"]
+        ensure
+          File.delete "lib/shard.cr"
+        end
       end
 
       it "initializes globs as string" do
@@ -102,7 +113,7 @@ module Ameba
       config = Config.load config_sample
 
       it "holds source globs" do
-        config.globs.should eq Config::DEFAULT_GLOBS
+        config.globs.should eq ["**/*.cr"]
       end
 
       it "allows to set globs" do
diff --git a/spec/ameba/glob_utils_spec.cr b/spec/ameba/glob_utils_spec.cr
index 41309195..fc47bacd 100644
--- a/spec/ameba/glob_utils_spec.cr
+++ b/spec/ameba/glob_utils_spec.cr
@@ -45,6 +45,30 @@ module Ameba
         subject.expand(["**/#{current_file_basename}", "**/#{current_file_basename}"])
           .should eq [current_file_path]
       end
+
+      it "raises an ArgumentError when the glob doesn't match any files" do
+        expect_raises(ArgumentError, "No files found matching foo/*") do
+          subject.expand(["foo/*"])
+        end
+      end
+
+      it "raises an ArgumentError when given a missing file" do
+        expect_raises(ArgumentError, "No files found matching foo.cr") do
+          subject.expand(["foo.cr"])
+        end
+      end
+
+      it "raises an ArgumentError when given a missing directory" do
+        expect_raises(ArgumentError, "No files found matching foo/") do
+          subject.expand(["foo/"])
+        end
+      end
+
+      it "raises an ArgumentError when given multiple arguments, one of which is missing" do
+        expect_raises(ArgumentError, "No files found matching foo.cr") do
+          subject.expand(["**/#{current_file_basename}", "foo.cr"])
+        end
+      end
     end
   end
 end
diff --git a/spec/fixtures/config.yml b/spec/fixtures/config.yml
index c5394b90..4cc49722 100644
--- a/spec/fixtures/config.yml
+++ b/spec/fixtures/config.yml
@@ -1,2 +1,4 @@
-Lint/ComparisonToBoolean:
-  Enabled: true
+Ameba/PerfRule:
+  Enabled: false
+Ameba/ErrorRule:
+  Enabled: false
diff --git a/spec/fixtures/source.cr b/spec/fixtures/source.cr
new file mode 100644
index 00000000..e69de29b
diff --git a/src/ameba/config.cr b/src/ameba/config.cr
index 821fa065..5030740c 100644
--- a/src/ameba/config.cr
+++ b/src/ameba/config.cr
@@ -55,10 +55,7 @@ class Ameba::Config
     Path[XDG_CONFIG_HOME] / "ameba/config.yml",
   }
 
-  DEFAULT_GLOBS = %w(
-    **/*.cr
-    !lib
-  )
+  SOURCES_GLOB = "**/*.cr"
 
   getter rules : Array(Rule::Base)
   property severity = Severity::Convention
@@ -95,7 +92,7 @@ class Ameba::Config
     @rules = Rule.rules.map &.new(config).as(Rule::Base)
     @rule_groups = @rules.group_by &.group
     @excluded = load_array_section(config, "Excluded")
-    @globs = load_array_section(config, "Globs", DEFAULT_GLOBS)
+    @globs = load_array_section(config, "Globs", default_globs)
 
     return unless formatter_name = load_formatter_name(config)
     self.formatter = formatter_name
@@ -239,6 +236,12 @@ class Ameba::Config
     end
   end
 
+  private def default_globs
+    [SOURCES_GLOB].tap do |globs|
+      globs.push("!lib") unless Dir["lib/**/*.cr"].empty?
+    end
+  end
+
   # :nodoc:
   module RuleConfig
     # Define rule properties
diff --git a/src/ameba/glob_utils.cr b/src/ameba/glob_utils.cr
index 71cd0cb5..df4e80bd 100644
--- a/src/ameba/glob_utils.cr
+++ b/src/ameba/glob_utils.cr
@@ -22,6 +22,7 @@ module Ameba
     def expand(globs)
       globs.flat_map do |glob|
         glob += "/**/*.cr" if File.directory?(glob)
+        raise ArgumentError.new("No files found matching #{glob}") if Dir[glob].empty?
         Dir[glob]
       end.uniq!
     end