diff --git a/lib/unit/system.rb b/lib/unit/system.rb
index ccd57e7..3783b39 100644
--- a/lib/unit/system.rb
+++ b/lib/unit/system.rb
@@ -60,6 +60,30 @@ def validate_unit(units)
       end
     end
 
+    # Parse a unit expression into a unit definition: an array of
+    # <tt>[factor, unit, exponent]</tt> triples suitable for +Unit.new+ and
+    # +validate_unit+.
+    #
+    # The expression is tokenized (see +TOKENIZER+) and evaluated with a
+    # shunting-yard pass over +OPERATOR+, supporting multiplication
+    # (<tt>*</tt>, <tt>·</tt>, or juxtaposition), division (<tt>/</tt>),
+    # exponentiation (<tt>^</tt>, <tt>**</tt>), grouping parentheses, and
+    # numeric literals.
+    #
+    #   system.parse_unit("m/s^2")  # => [[:one, 1, 1], [:meter, ..., 1], [:second, ..., -2]]
+    #
+    # An empty or whitespace-only expression has no tokens and yields the
+    # *dimensionless* unit <tt>[]</tt> — the same definition produced by
+    # <tt>Unit(1, "")</tt> — so conversion stays consistent with construction
+    # rather than returning +nil+.
+    #
+    #   system.parse_unit("")    # => []
+    #   system.parse_unit("  ")  # => []
+    #
+    # Unrecognized glyphs survive lexing (see +SYMBOL+) and fail loudly as an
+    # "Undefined unit" +TypeError+ during validation rather than being dropped.
+    #
+    # Raises +SyntaxError+ on unbalanced parentheses.
     def parse_unit(expr)
       stack, result, implicit_mul = [], [], false
       expr.to_s.scan(TOKENIZER).each do |tok|
@@ -87,7 +111,7 @@ def parse_unit(expr)
         end
       end
       compute(result, stack.pop) while !stack.empty?
-      result.last
+      result.last || []
     end
 
     private
diff --git a/spec/error_spec.rb b/spec/error_spec.rb
index 71747e7..d0e55a1 100644
--- a/spec/error_spec.rb
+++ b/spec/error_spec.rb
@@ -26,4 +26,27 @@
     end
   end
 
+  describe "converting to an empty (dimensionless) unit string" do
+    it "treats #in(\"\") as the dimensionless unit" do
+      expect(Unit(1, "").in("")).to eq(Unit(1, ""))
+    end
+
+    it "treats #in(\" \") (whitespace-only) as the dimensionless unit" do
+      expect(Unit(1, "").in(" ")).to eq(Unit(1, ""))
+    end
+
+    it "treats #in!(\"\") as the dimensionless unit" do
+      expect(Unit(1, "").in!("")).to eq(Unit(1, ""))
+    end
+
+    it "treats #in!(\" \") (whitespace-only) as the dimensionless unit" do
+      expect(Unit(1, "").in!(" ")).to eq(Unit(1, ""))
+    end
+
+    it "raises a clean TypeError (not NoMethodError) from #in! for an incompatible unit" do
+      unit = Unit(5, "m/s")
+      expect { unit.in!("") }.to raise_error(TypeError)
+    end
+  end
+
 end
diff --git a/spec/system_spec.rb b/spec/system_spec.rb
index c5bcd2b..32ed18b 100644
--- a/spec/system_spec.rb
+++ b/spec/system_spec.rb
@@ -172,4 +172,16 @@
       expect(Unit(1, "MeV", system).unit).to eq(Unit(1, "megaelectronvolt", system).unit)
     end
   end
+
+  describe "#parse_unit with an empty or whitespace-only expression" do
+    before { system.load(:si) }
+
+    it "returns [] (the dimensionless unit) for an empty string" do
+      expect(system.parse_unit("")).to eq([])
+    end
+
+    it "returns [] (the dimensionless unit) for a whitespace-only string" do
+      expect(system.parse_unit("  ")).to eq([])
+    end
+  end
 end