diff --git a/src/block.h b/src/block.h
index cd3a48e..13daea1 100644
--- a/src/block.h
+++ b/src/block.h
@@ -25,13 +25,18 @@
 class block {
  public:
   block(): line_bytes_(0) {
+    lines_.reserve(max_lines_per_block_);
+  }
+
+  bool empty() const {
+    return line_bytes_ == 0;
   }
 
   void add_line(std::vector<uint8_t> &&line) {
     assert(!line.empty());
     assert(line_fits(line.size()));
     line_bytes_ += line.size();
-    lines_.push_back(line);
+    lines_.emplace_back(line);
   }
 
   bool line_fits(unsigned size) {
@@ -40,7 +45,7 @@ class block {
   }
 
   void flush(FILE *f) {
-    if (line_bytes_) {
+    if (!empty()) {
       fprintf(f, "%dw%c%c",
               line_bytes_ + 2, 0,
               static_cast<int>(lines_.size()));
diff --git a/test/test_block.cc b/test/test_block.cc
index 0b39a30..5c840b0 100644
--- a/test/test_block.cc
+++ b/test/test_block.cc
@@ -24,7 +24,20 @@
 typedef std::vector<uint8_t> vec;
 
 const lest::test specification[] = {
-  "Block line limit",
+  "A block is created empty",
+  [] {
+    block b;
+    EXPECT(b.empty());
+  },
+
+  "Adding a line makes a block no longer empty",
+  [] {
+    block b;
+    b.add_line(vec{1});
+    EXPECT(!b.empty());
+  },
+
+  "A block can contain 128 lines",
   [] {
     block b;
     for (int i = 0; i < 128; ++i) {
@@ -34,7 +47,7 @@ const lest::test specification[] = {
     EXPECT(!b.line_fits(1));
   },
 
-  "Block size limit",
+  "A block has a size limit of about 16 kilobyte",
   [] {
     block b;
     for (int i = 0; i < 16; ++i) {
@@ -44,30 +57,33 @@ const lest::test specification[] = {
     EXPECT(!b.line_fits(400));
   },
 
-  "Flush",
+  "Flushing an empty block does nothing",
   [] {
     block b;
-    {
-      tempfile f;
-      b.flush(f.file());
-      EXPECT(f.data().empty());
-    }
-    for (uint8_t n = 0; n < 10; n += 2) {
-      vec line = {n, static_cast<uint8_t>(n+1)};
-      EXPECT(b.line_fits(line.size()));
-      b.add_line(std::move(line));
-    }
-    {
-      tempfile f;
-      b.flush(f.file());
-      EXPECT(( f.data() == vec{'1','2','w',0,5,0,1,2,3,4,5,6,7,8,9} ));
-    }
-    {
-      tempfile f;
-      b.flush(f.file());
-      EXPECT(f.data().empty());
-    }
+    tempfile f;
+    b.flush(f.file());
+    EXPECT(f.data().empty());
   },
+
+  "Flush() writes the lines to a file with a proper header",
+  [] {
+    block b;
+    for (uint8_t n = 1; n < 6; ++n) {
+      b.add_line(vec{n, n});
+    }
+    tempfile f;
+    b.flush(f.file());
+    EXPECT(( f.data() == vec{'1','2','w',0,5,1,1,2,2,3,3,4,4,5,5} ));
+  },
+
+  "After flush() a block is empty again",
+  [] {
+    block b;
+    b.add_line(vec{1});
+    tempfile f;
+    b.flush(f.file());
+    EXPECT(b.empty());
+  }
 };
 
 int main() {
diff --git a/test/test_line.cc b/test/test_line.cc
index fd89c57..38ab10e 100644
--- a/test/test_line.cc
+++ b/test/test_line.cc
@@ -53,7 +53,7 @@ const lest::test specification[] = {
   [] {
     EXPECT(( encode_line(vec{1,2,3}) == (vec{1,sub(0,2),1,2,3}) ));
   },
-  
+
   "Encoding a (non-initial) blank line",
   [] {
     EXPECT(( encode_line(vec{0,0,0}, vec{1,2,3}) == vec{0xFF} ));